Honestly, it's just bad luck. That's simply not how matchmaking works - you don't get matched only with B1 because you are B1, or only with B2 because you are B2. You get matched based on your hidden matchmaking rank. You still get the same games that you would after the same wins and losses regardless of rank. Some people get failed series or quick demotions out of bad luck often enough to be regularly matched with/against people several ranks higher (a full division would be extreme, but 1-2 levels isn't). If you keep win more than you lose your MMR will go up. If your MMR goes up, you gain more LP for wins and lose less for losses - it's even if you have the average MMR for you rank, disadvantaged if you have a lower MMR than your rank, advantaged if you have a higher than average MMR for your rank. Better MMR = easier climbing (if you can still win).
I have to remind myself of this daily, being at the bottom of bronze. I climbed out of bronze 5 winning 4 out of 5 games since placements, and then got demoted again only because in bronze 4 I lose 21 LP or gain 10 and had a slightly unlucky streak. In Bronze 5 I'm even (15/15). But as long as I keep winning my MMR will go up and next time I promote maybe my LP will be more even and I won't get demoted again.
(I'm only winning 2 out of 3 now though, maybe I'm on tilt, heh. I was almost certainly on tilt in placements where only won 3 out of 7 and one of those wins was purely being carried... or maybe it was just the craziness of season opening. Either way I trashed my MMR and my rank.)