Electrical – Why does the breaker keep tripping on a circuit with two rooms’ lights and a sewer pump

I'm going to take a wild guess that the previous home owner put the blanks up because they had the same problem and didn't want to fix it. You likely have a short circuit and need to track down where the short is occurring. I'd remove the bulb from the fixture, unhook each of the wires that you connected before, and use a multimeter to test for "continuity" between the wires on each run (e.g. hot to neutral, hot to ground, neutral to ground for the line to the switch, for the light fixture without a bulb, and anywhere else that was previously disconnected). If you don't have continuity with a simple low voltage tester, then the problem could be arcing that only occurs when 120V is applied. Either way, get this fixed before turning the breaker back on with this fixture wired.

If you're referring to the circuit breaker switch being in the middle after it's been tripped, that's just the "tripped" position. As far as I know it doesn't tell you anything about the reason.

You have arc fault breakers, which means the breaker was tripped because:

1. there was too much current flowing
2. there was an arc fault.

Based on the relatively low loads and intermittence you describe, I would guess it's an arc fault issue. Perhaps a faulty wiring connection. Maybe it's being jostled occasionally if someone walks nearby and causes just the right vibration or something. Or as Steven mentions in a comment below, a faulty switch could cause an arc fault.