BLDC low-side gate oscilations
It's likely caused by drain-gate capacitance and you won't see it on your upper N channel MOSFET because its drain is connected hard and fast to your DC bus voltage. The lower side N channel MOSFETs have their drains at the switching node and, under these circumstances, when they start to switch on, the drain voltage rapidly starts to fall and that action, in turn, hits the gate (via the inter-capacitance of the device, and causes the gate voltage to reduce (and sometimes oscillate) or wobble a bit.
If you reduced R17 down from 56 ohm, you might see an improvement provided that the MOSFET driver chip (MCP1402) can handle the peak current into the gate changing that rapidly at the switching point.
Basically it's miller capacitance and negative feedback.
Here's a picture of MOSFET turn on transition with insufficient gate drive: -

Improved response with bigger drive capability: -

Pictures from here.
It's also not helped much by having the shunt resistor in the source circuit - as soon as drain/source current flows, the shunt voltage rapidly rises and slightly switches off the lower N channel MOSFET. It's probably a bit of both things in reality. Both are negative feedback actions.