Project resets when powered via Vin, but not via USB power. Why

Hi,
My project resets itself when I press my "Go" button and it is normally powered. But, when the Uno is receiving power via USB everything works fine. What might cause this reset? A voltage dip to the Vin pin? A voltage spike?

I have a 24V 0.6A solenoid connected to an Uno through a MOSFET. There is also a 5HP motor contactor switch that is energized via current to a relay. The Uno is powered via a 7.5V switching power supply(step down from 24V). There are flywheel diodes on both the solenoid circuit and the relay circuit, and I have measured 7.5V on the Vin pin.

Any thoughts on why the system behave differently under the two scenarios?

Thanks, Sam

Perhaps the 24V rail is dropping a lot when the contactor is switched, and the SMPS cannot maintain the 7.5V high enough for Vin during the transient, leading to a drop in 5V and a reset? This might only be for a tiny fraction of a second.