Do you check / monitor for neck tension each time you load up a batch?
Have you changed anything in your production process (new dies, powder thrower, case lube used etc)? We’re your MV measurements taken previously and this most recent +100 fps batch from the same shooting location and roughly similar conditions?
how many rounds did you chrono and what were the velocities? SD/ES? Difference in SD/ES between this last batch and previous batch?
Rounds sitting for a couple months won’t change or be any different from when first produced unless they were not stored in a room temp, dry location. Were they?
Carbon ring does seem like a good explanation but could be other things.
ETA - if you use a charge master or other electronic scale, do you occasionally compare thrown charges between the two to confirm they align?
Once I discovered my Lyman gen6 was under throwing by .3-.4 grains compare to two of my charge masters and it just needed recalibration. Just throwing it out there as another possibility, esp if your MV is consistent ~100 fps faster round over round all of the sudden compared to all your other data taken previously.