My pup does that! For him, it's because his aim is bad. He'll get all feet on the pee pad, cantilever his little butt over the edge, and think that's good enough.

We ended up getting a set of machine washable bathmats to rotate through, for next to the pee pad.

(he's an old pup, so his bladder no longer lasts the night and no one wants to do 3 am walkies. He's also got doggie dementia, so I'm not worried about training him to have better aim)