There are a few things that could lead to this, my personal standby is this mindset, we had 3 tiers of ranking (Diamond, Master, and Challenger(ed)) basically get erased when the soft reset came in because nobody could place above Plat 1. This means that we had a previously 7 tiers fitting into 4, giving everyone the appearance of decaying with the reset because our ranking is tied in with how we are performing in comparison to everyone in NA. This explains a 2-3 division drop (B1 to B4 or G5 to S3). Then you also have to add in the fact that because you weren't able to play you could not get better or worse so you stagnated. This allows people who were previously worse than you to improve past you much quicker than normal which would drop you about 1 more division at most. Finally you also have to account for your opponents in your wins and losses for placements(ie. having 2 wins vs S4's and 3 wins vs B5's but 5 losses to B3's would put you at about B4). These can all lead to decay or even improvement in ranking after placements but with improving being under special conditions (going 10-0 or 9-1 vs opponents who are a full tier above you). These circumstances that cause the seemingly over night decay from S5 to B4 is also an intentional act by Riot, from my viewpoint. This decay forces players to not only play their placements but also to try and get to where they were before the reset. In the end though don't lose heart, because lets say you end up going from G5 to S5 (me), it is possible to skip whole divisions if you show that you're at that level.
GL man and like I said don't let it disturb you, if you can make it out of Bronze once, it gets easier the second time around :)
