It's well know that as a person with clinical depression begins to recover, i.e cease's being catatonic & regaining some motivation, that looks like recovery but is in fact the most dangerous time as the person still has the same thoughts & now has the energy to act on them.
Some of it is down to the modern anti depressants as @AngelGirl said they are to a depressed person, well me anyway amusingly able to increase suicidal thoughts, though that is usually at it worst in the first 2-4wks & more so for some reason if the person is under 25.
I have been around sadly long enough to predate all modern anti d's, pre Prozac you had the choice of Tricyclic or MAOI's (which were nasty things that you oddly couldn't eat cheese with without serious side effects!!) I have seen my notes from that time & twice they thought i had turned a corner only to find it backfired on all of us. So yes modern Ad's have an effect but it is also a feature of clinical depression sadly.