Go with immaturity, even though that's not exactly true (alcoholism DNE immaturity, at least not real alcoholism).
If you say you had a serious health issue, schools may think to themselves: "If we admit this guy/girl, what happens if they get sick again? Obviously the last time they were ill, it was a two year ordeal. It was something that encompassed their whole life, to the point where they almost flunked out of school"
Illnesses are many times chronic. A school could be afriad of a relapse, like I illustrated above. 9 times out of 10, though, immaturity is not something that one reverts to. They will see you obviously turned your life around for the better. For this reason, I think going with immaturity as your excuse will be a stronger case.
BTW, are you doing an SMP? With 2 years (roughly 60 credits?) at a 1.5 followed by 60 credits at a 3.95, you have a cum. gpa somewhere around a 2.7, correct? Chances are slim with those stats, regardless of a trend.