After consuming about 3/4 of a batch of brown ale, the remaining seem to be over carbonated. I have been brewing for about a year and a half and have not had this problem before. What could cause this?

Could it be that the last of the brews are just now fully conditioned and were over primed for the temperature of the beer at the time of bottling?

I assume you are referring to a bottle conditioned batch. How long has it been bottled? How was the carbonation level a month after bottling?

If you did not stir your beer often during the bottling process, it’s possible you got some bottles that had a higher concentration of priming solution.

When you say they’re over carbonated, do you mean the bottles gush? If so, could be an infection.