When the soft reset takes place, your hidden MMR value has a considerable drop. Nothing like Platinum to Silver, but I think around enough to drop a tier. When you play your placement matches, your MMR is affected more drastically by your first ten matches, which is how you get placed, followed by some 10-20 games following, if I've been following the system correctly.
In other words, your MMR dropped because of the soft reset, and your subsequent games will have more impact on your rise/fall in the divisions. If you're level of play is Silver 4 quality, then besting those in Bronze 1 - Silver 4 shouldn't be a considerable challenge, and you'll reach where you were at the end of last season within a few games.
Hopefully this helps a little :)