Sometimes you'll scoop out all bubbles, then come back a while later and see a bunch more in one spot on your piece, usually on one edge or corner. That means there was a larger air bubble trapped behind your paper, and as the resin starts to set up, that bubble makes its way up and around the edge of the paper in a stream of smaller bubbles, almost like a fountain.
To prevent this from happening, carefully lift up your mold and check below your paper to see if there is a bubble hiding there (keep your mold level so the resin doesn't flow out). If you see a bubble, you can either press the paper down against the bottom, forcing the bubble up all at once, or you can lift up the paper, allowing bubble-free resin to flow beneath it, then gently push the paper back down. Allow bubbles to rise and pop.