Possessive 3yo

So we go through stages of this and I still cant exactly pinpoint why he goes through this. We teach and enforce respecting one another space and feelings along with sharing and not taking one another's toys. My oldest is almost 4 and my younger son is 2 and a half. Well the 3yo go through stages where he doesnt want the 2yo to play with ANY toys. He will hoard them all as well as intentionally taking the "special" toys from the 2yo. (Like his sleeping babies etc) there are plenty of toys for each to play with and like I said we make very sure that theres no stealing of toys going on behind our backs bc when this first happened I figured that was the root but thus far it really doesnt happen. I've even hidden an watched many times when they think they arent being watched