Steam coming out of freeze plug?

So I was out plowing snow with my 1978 1700 2 cyl diesel when suddenly coolant started blowing out the rear block freeze plug and started running a little warm. Headed back to the barn - and sure nuff steam and coolant was pouring out the rear plug that was slightly unseated to the right side. so now for the questions:
1) what caused the plug to become unseated? would being hot do that on its own? Note - the coolant is very fresh and full and a good green color. It is stored in a barn and i would guarantee the coolant in the block did not freeze even partially.
2) Normally i can plow snow and the block is just warm - this time it was hot. the top radiator hose was pretty warm so that tells me the tstat is opening. the bottom hose was cooler but warmer than the air outside. The best i can tell the water is circulating but couldn't spend much time on it yet to know for sure.
3) as far as repairing the plug - buy new or re-use existing? also any ideas on the proper way to re-seat it?
4) OK to continue to plow if i make sure it doesn't overheat?

Thanks for any info you can provide - i enjoy reading the posts - the knowledge shared is really incredible.

Thermostat could have stuck, if only temporarily. Plug could have gotten old. I would use new plug. And certainly wouldn't run it much if it's blowing coolant very bad.

Usually a pretty easy fix in the rig. I have done plenty over the years. Use a punch and hit it on the top end to punch it so it lays perpendicular to the block then come in from an angle and put the punch end on the lip and knock it out. Use a good new brass plug and a good freeze plug glue or sealant (the brown sticky stuff)

She if you can find a freeze plug tool to install the new one. Normally people will fab one up but they do sell them. They look like seal drivers and fit just right into the diameter of the plug. You clean up the gunk on the plug hole from whence the old plug came out then slather the new outside perimeter of the new plug with a good freeze plug glue then drive er on home!

Thanks for the responses. follow up question - when putting in the new plug does the block need to be chilled? I've heard one way
to do it is to take a can of compressed air and when the can is upside down it will spray super cold air. this is used to chill
the block, then install the plug so that the plug will expand when it warms up. Worth it?

I still want to verify the water pump is really working - watching for circulating coolant is the only way i know but am open to
other ideas. Thanks .

I'd rather chill the new plug and let it come to a matching temperature on the block. It'll be an easier fit if it isn't the bore that's chilled.

Personally, I wouldn't be concerned about manipulating the temperature on either piece.