This is a release of gases.
CO2 is produced during the fermentation. The colder the beer the more of this CO2 it will hold, and as it gets warmer it will release it.
There are two reasons it might be happening:
1. If the temperature of your beer has warmed up, even 1/2 a degree, it will release CO2 that it was able to hold at cooler temps.
2. It is also possible that your fermentation stopped and then restarted, but I doubt it.
In Process - Russian Imperial Stout, Nelson Sauvin Rye IPA, Mild No.3
In Kegs - Barley Wine, Apfelwein, Wild BlackBerry Wheat, Coffee Oatmeal Porter
Gone - so many :(