If the motor gets disconnected while the driver outputs are enabled (even at idle when it holds position), then the driver has most chances to burn. Maybe the connector of the X motor has a bad spot, gets heated at some point and then doesnt make electrical contact properly. Or maybe you disconnected the motor while driver outputs were enabled. After some time of with no movement, the outputs get disabled, there is a setting in firmware which changes that period. Also could use a button like "motors off" and that (should) get their outputs off. But its much safer just to disconnect everything and wait few seconds for all caps to discharge, and do motor changes only with when electronics are *not* powered.
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