Grant Erwin writes:

You are damn lucky your mill got home OK. Two of your 3 tie-downs
relied solely on the position of the knee for tension. Bridgeport
knees have been known to work their way downwards while being moved,
which apparently didn't happen to you.

I neglected to mention, the knee was locked. And only 1 of the 3 depended
on it. But you raise a valid point.