Difficult to say, as from a pile of glass scraps you can't really do any detective work. It could be both, or a combination.
I've never heard of e9 windows breaking, so i'd say it has to do with at least some part of the work.
Did it break when it reached fully down? or fully up? or during the travel up/down?
The motor could be too powerfull, pushing to hard on the glass. But as Porsche windows aren't exactly thick guage (i expect; too heavy) and i think smaller in size vs the E9, I expect the posibilty of the porsche motor having too much power wasn't the culprit.
Now installing the bolts incorrectly may just do the trick. I know for sure there are a few different length bolts in the whole door. At least 20mm and 12 mm M6 ones are used in the bottom window guide bracket), and swapping those could possibly create an obstruction. Other bolts holding the door opener, armrest and/or ashtray could also be problematic.
Third, the window slide mechanism has at least one rubber end stopper on the bottom as a defined end point. I do not know the motor-to-gear-to-glass setup of E-windows (i have manual), but i expect that there are 2 rubber buffers as well mounted on the large gear wheel as end stops. If any of those 3 are missing / incorrectly installed or adjusted I do not know what would happen ( I never tired ofcourse), but they are there for a good reason. Our E9's are quite well engineered (apart from the rust part).
all in all, there are a few things to check before pushing that button again.