I know about P90 hum, I've had P90 for years, I embrace it, it's part of the tone.
So, why is it when I play (just did to check again) my cheapo $84 Epiphone LP Special with P90, it keeps the same normal P90 hum when I let go of the hardware. The SE in the same amp, same cable, same angle/distance to the amp, same lights on, same cat roaming around (and a bird just hit the sliding glass window ... boing ... really !) ... so, the SE in the exact same situation, buzzes like a swarm of buzzers.
Ground between bridge and electronics checks with a multi meter.