Locked starter question....

[41panelmark]

So I have heard of this happening before many times and I have seen posts about it here on the barn a lot of times, (locked up starter).

Went to get a load wood today, pulled up to the scale house shut off the engine and could not get it restarted a few minutes later. Engine was locked up solid.

So I loosened the bolts that hold the starter in and got a short tow in order to let the clutch out and get the engine freed up. It worked, hit the button and it started fine. Tightened up the starter and used it like normal the rest of the day.

What causes this and why does it seem to affect old flatheads more than others?

thanks guys.