Some sins, due to their quality, leave no opportunity to repent and will lead their committers to the irreversible path of Hell, the same way falling from heights without employing a parachute or landing equipment will lead to the disintegration of the body or the same way falling into a deep sea without possessing the ability to swim will naturally lead to drowning and in both examples there is no way back. Of course we must realize that the term "automatically" is not precise in this regard, because we all know that such a punishment is the direct result of such a decision.
Before answering the abovementioned question we must pay attention to what is meant by "automatically" and whether or not we would use this word for issues that are the direct result of one's behavior and conduct. For example would we say that if a person places his foot in fire, his foot will "automatically" burn?! Or would we say that if someone jumps from heights he will "automatically" hit the ground and his body will be demolished?
Having said this introduction, we will explain the answer to your question.
After creating man, Allah has declared some acts forbidden and mentioned that if such acts are committed, Hell will be the destiny of the sinners.
We know that some sins are like spoiled food whose long term consumption is a threat to one’s health, but others are like devastating explosions which destroy and demolish one's spiritual life in an instant.
At the same time due to Allah's great mercy there are many paths, which by taking them, people have the opportunity to nullify some of their heavy and severe punishments or at least reduce them to some extent.
Making up for unjust damages, asking forgiveness from those affected from the damages of certain sins, and finally, repenting to Allah are the steps of compensating for a sin.
Obviously these compensating steps and specifically repenting should be done before one's death and while he is still alive and consequently, a repentance that is delayed to the final moments prior to death or after death will have no effect.
In the light of what was explained we will return to the sin of suicide that was mentioned in your question.
In a hadith by Imam Baqir, he says: "A Mu’min (believer) may suffer from catastrophes of all kinds or die by various incidents, but he will never commit suicide."
In this regard Imam Sadiq says: "Whoever commits suicide deliberately will remain in Hell forever."
In some ways it is correct to say that committing suicide is a sin that automatically entails one to be taken to Hell, but why?
1- As we mentioned above there is an opportunity to nullify or reduce the punishment of sins through repenting to Allah and the repentance should be done before one's death, but committing suicide is a sin that denies one the opportunity to repent. In other words, by committing this sin one has declined his last chance to nullify or reduce his punishment, thus, naturally entering Hell.
2- Life is a precious gift that Allah has granted to man. This gift is actually an opportunity with the help of which one can achieve the greatest levels of perfection. The person who commits suicide is virtually blowing his precious opportunity and being punished in Hell will be the direct consequence of declining this opportunity. The same way if someone all of a sudden throws himself into fire; his body will burn as the direct result of his inappropriate action.
Also, the same way committing suicide is an unforgivable sin, taking away others’ opportunity to live will entail an inevitable punishment.
Regarding this issue Imam Sadiq says: "A Mu’min (believer) will have the chance to make up for his behavior and conduct, unless he takes part in killing another believer unrightfully, and one who kills a Mu’min, (even though it seems that he has the chance to repent but certain events will take place that he) will not actually repent."
Despite the fact that some sins do not completely take away the chance to repent, but they reduce the possibility of repentance. Let's examine the following hadith:
Imam Sadiq was asked about an individual that consumes alcoholic beverages and what his (spiritual) situation would be like? The Imam responded: "His Salat (prayer) and also his repenting will not be accepted for 40 days and if he passes away in this period of time he will enter Hell."
In closing, one can conclude that some sins do cause people to enter Hell automatically, but this should not be understood in a way that leads to belief in Jabr (Force and predetermination) and one should not take the responsibility of the sinner in regard to his sins lightly. In fact, this automatic entering is the direct result of the sinner's actions, the same way the automatic demolishment of a person's body who throws himself from a height does not contradict his responsibility for his act. However, according to the viewpoint which suggests the manifestation of actions or the unity of action and reward/punishment, Hell and Heaven are the manifestations of our own actions, meaning that it is the person who creates Heaven or Hell through his actions, not that he has done an act and according to an arrangement (with Allah) he will now be rewarded or punished by going to Heaven or Hell. From this point of view the term "automatically" will lose all meaning regarding sins.