Weil McLain condensate trap accumulating water

I have a Weil McLain gas boiler. Every few weeks, it fails to complete the startup cycle and throws a pressure switch error. When this happens, the internal condensate trap is full of water (the side of the trap where the white tubes connect). If I empty the water out of the trap, the furnace starts fine.

Here is a picture of the trap that I'm referring to (without water, I just drained it this morning)



Does anyone know why the water accumulates in the trap, and what I can do to prevent it?

Thanks!