N-mosfet low-side switch being turned on with no direct gate drive signal
SHow a photo of your wiring.

With 20 amps switching in 20 nanoseconds (assume that), inductively coupled across 1cm distance to a 10cm by 1cm loop, the induced voltage is

Vinduce = 2e-7 * Area/Distance * dI/dT

Vinduce = 2e-7 * 1cm*10cm/1cm * 1e+9amp/sec

Vinduce = 2e-7 Henry/meter * 0.1meter * 1e+9 = 2e^(-7 -1 +9) = 20 volts

How about some photos of your wiring.