I thought this was a control card problem as my Sieg C2 is 2016 vintage with the brushless motor, no high/low ratio and a complicated motor control card.
Turned out to be a loose connection on the forward/reverse toggle switch. This is this is an input to the control board, not the older multipole cleverly wired switch that reverses the power to the motor on its own. Its a two pole switch. Closed contacts for normal rotation, open for reverse. If you get a loose connection, the lathe will stop then reverse unexpectedly, probably ruining your work piece and tool.
Hope this is helpful to other Sieg C2 users.