I think you should read your manual. You hit Reset when a tapping cycle was active. I believe when this cycle is active the machine completes the cycle then responds to the Reset.
You should have pushed Estop before getting into the machine. This would have disabled all the drives. Your machine was simply stalled but under power and as soon as you loosened the collet it was able to complete the cycle and stop.
Incidentally if you have the 10,000rpm direct drive spindle, in other words no gearbox you were pushing it trying to drive a 5/8" tap. These machines do not have a lot of torque and it is easy to stall the spindle on taps larger than 1/2" especially NC threads. The way to tap larger sizes is Repeat Rigid Tapping and just peck in 2 threads at a time or less.