I think it's because you have two of them (sigh)
It also started to happen to me when I had two def.walls, but doesn't happen when I field only one. The logic (pardon me if the logic seems empty, I'm trying to provide meaning to something which, if true, has very little) is perhaps that if you have more defenders that want at all costs to be in the wall, then you have less defenders to mark opponents, an so are more at risk if the ball bounces on the barrier.
I think many SA's have this kind of balancing properties, where two players with the same SA for one reason or another don't give you much more than a single player with that SA. I still have to figure out details/theories thoughHowever, be warned that not many people agree with me.
For FK, CK, PK I think this is not the case, you can have any number and they don't interfere with each other. As many will tell you though, PK seems rather useless.