Happened a few weeks ago and again yesterday and today.

Very bottom of the lats. Sore as 87 MFers.

Anybody else ever experience this?

You use your lats when you pull your arms straight down from over your head, they contribute heavily to pulling straight armed towards your belly.

It's the reason that pullovers hammer the lats.

Yep, they'll work your lats a little.  Deadlifting works you lats a little bit, for the same reason.

You're lats stabilize your upper body as you move through that range of motion holding the roller. You're not used to using them in that role.

Same thing happened to me in the Army. Doing PT, you would do pushups, stop and rest. During your PT test, you had to rest in the front leaning rest. That requires the mid back muscles to stabilize your upper body. That doesn't happen when you train and rest on your knees. My upper back was always sore after a PT test.