Spindle shot off in the Y direction half way through carving

I was carving the pocket portion of an inlay test. The pocket was going to take three passes. Part way through the second pass, the spindle just took off in the Y direction cutting right through the middle of the partial carving. I was nearby and heard the change in pitch and hit the stop button. Clamps were tight and the G-Code simulation looks good. I was using the free version of Easel connected via USB cable. Any idea what would cause this. (Using 1/8 inch end mill bit on soft pine) Thanks!

Generally see this if there was a communication error or USB failure - you have all the power saving features shut off, no screen saver, etc?