Almost all of my brushes are bendybrushes. They all end up that way. I have no idea what I'm supposed to do to prevent it. I didn't even know it was possible to prevent it. Like M. Heisler, I actually prefer it for most things.
I consider it part of the life cycle of a brush. First it's normal, then it's a bendy brush, then it becomes a drybrushing dusterbrush.