And to what extend it is related with other more long term factors such as stronger barbarians, or change of roman economy unrelated to actions of the Severans, or Plague of Cyprian, the rise of the Sassanids or other factors.
Actions I am thinking about is Severus expansion of roman legions, destroying Albinus troops and both Severus and Caracalla debasing the currency for increasingly large payments to roman soldiers. Could this have started a death spiral in terms of discipline of the legions, and also economic development? We know the debasement causes important inflation problems. We also know that Macrinus have trouble keeping with the commitments of Caracalla for increased pay of troops.
Also interesting, every single individual who ruled as emperor and is usually called as part of the dynasty, ended up murdered by some kind of roman soldiers. Other than Severus. Whether it was Caracalla and praetorian guard, Macrinus by rebelling troops in Syria (which he refused to increase their pay), Elagabalus by Praetorian guard, and Alexander Severus by men of his legions.
In Alexander's case, there seemed to be a decline of discipline and attempted many mutinies in his eastern campaign, praetorians rose in mutiny in the streeets of rome against harsher rules of discipline
Indeed Elagabalus and Alexander, show a trend we also see in later byzantine period, which is mother regent and young king (or emperorship passes to one who marries the heiress), rarely ends with good results. Perhaps part of this is unsuitability for war leader for mother and young son, younger kings not being as able, another might also have to do with the society not liking a female leader and opposing them due to stereotypes or dislike seeing women in position of power.
Though young king in itself also is usually not that good.
What I see is that the institution of discipline which the romans have cultivated and used previously did decline during the severan dynasty, and I think it does have something to do with the Severruns actions, and in fact Septimius shares the blame along with of course Caracalla.
Also, add the tyranny and murders of Caracalla, the huge incompetence of Elagabalus, and then there is Alexander and his mother. They were trying, but for the role of being a general, and acting based on roman standards, they weren't suitable. Though part of the problem, has to do with this kind of behavior by the legions.
Anyway, I think the long civil war that followed, was related to only the increased Sassanid threat, but a loss of dynastical prestige. You have the rise of people like Thrax attempting to be emperors by killing Alexander. Again to be fair, the troops themselves, who chose this guy, are very much responsible, and if discipline is declining isn't necessarilly only due to the actions of the emperor.
Anyway, what does Maximinus Thrax do? He doubles the pay of soldiers. Increased pay, necessitated increased taxes as well.
And with Severus death, Thrax rise, and rise of other emperors, and a period of crisis, many wannabee emperors, lack of unified empire, we have the crisis of the third century.
In my view, institutions and actions taken by leaders in guiding those institutions matters. Isn't only thing that matters, but it does. At such the Severans, and some that followed, played a role in the rise of easier and quicker rebellion and break in discipline. I would put more of the blame on the first three (or four if we include Macrinus) than Alexander and his mother. I am not convinced the crisis was inevitable. Of course more troubles due to plague and rise of strong enemy were going to be there, but I see a transformation to the roman society that becomes more unstable, prone to usurpations and end of emperor through their own troops. Though Roman empire always had that problem. Perhaps there other factors at play influencing that beyond the actions of the Severans.