Breaker trips with seemingly light load

I have a circuit that I have mapped to 2 indoor outlets (both empty) and two outdoor outlets (both covered from weather). One of the outdoor outlets is a GFI (nothing plugged in) and the other has a security camera plugged in. The security camera uses DC power in the form of a USB cable with a small USB transformer cube plugged into the outlet. After working fine for several months, the camera stopped working and I realized the outlet had no power. A bit of troubleshooting let me to discover that the GFI outlet had tripped, and everything worked when I hit reset on that. But then after about an hour, it tripped again. This happened a few times and I suspected a bad GFI outlet because there's really no load on the entire circuit other than the little USB transformer for the security camera. So I swapped out the GFI for a regular outlet, and again everything worked for about a hour. Then the actual breaker tripped. Because both the GFI and the breaker tripped, I now think there is something shorting or surging on that line. Is it possible that the little USB transformer is causing this problem? The weird thing is that the camera works fine for a while when the breaker is reset, so I know the transformer isn't blown. My next move is to replace that thing, but wondering if anyone has seen a problem like this before.