I think that I have explanations for both of these 
This happens when the gcode is inches. The gcode file leaves the machine in inch mode but the jog distance is still 100 so it tries to jog 100 inches. We need to come up with a better fix for that.
Whenever any motor is moving all of the motors will hold position which leads to the belts tightening up. I’m open to changing how this works but we need to come up with a clear definition of what the expected behavior is