A tree frog burrowing can often mean the humidity is too low, the temperature is too low, or it is afraid and needs a place to hide. I have no experience with a big-eyed treefrog though. I understand they're from Africa, and for that reason alone I would not house it with a leopard frog.

Here's another care sheet for the fairly aquatic leopard frog Frog Forum - Leopard Frog Care - Rana pipiens (Schreber, 1782) and R. sphenocephala (1886)