Why is silicone caulk "falling" or peeling away from new bath countertop and wall?
This looks like something is shifting and pulling the caulk off the wall. My guess would be the vanity top shifted a bit on top of the vanity cabinet due to not being secured enough to the vanity.
Silicone caulk shouldn't be applied over old silicone caulk. Your contractor should have removed all the old caulk and installed all new caulk. This is something you can and should do.