Why did my topcoat bubble?

[redhexagonal]

I put yellow grit on my wall and a layer of bonding on the lower half to level it (it curved) . I PVA’d the wall next day 2:1, twice then did a couple of coats of multifinish. The section over the bonding bubbled alot after the 2nd coat, and I had to wait ages to get rid of the bubbles at all, and its still not really smooth. The section above was perfect.

I guess I sand it , but why is this happening? I got these bubbles last time I used multi over bonding too.