Loftar has stated before that if anything happens to be blocking their growth at the moment they would grow, they will never try to grow again. My guess is something or someone happened to be in the way at the exact right time.
The lower mulberry is waiting for decay hits from the server, that's how trees grow to their last stage. The upper tree seems to have been blocked by something when its growth timer clicked in.
Edit: woops, I forget how fully grown mulberries look like. The upper tree is the one waiting for the hit, the apple trees probably did just get blocked in the middle of their growth cycle
Wait, does it consider it's growth blocked if something is within an area as big as the leafs, or if it's to the point where not even your character can go any further?
trees start off taking 1 tile, as they're growing they start needing more tiles around them until they take a 3x3 area, if that area is blocked when they attempt to get to the next stage they become stunted and stop growing.
The very last stage of trees requires a server decay hit, so its not bound by a timer, only luck.