It could be finishing up fermentation, but it could also be the co2 in the beer coming out of solution. Cold liquids "hold" co2, and when it warms up, the co2 is released more readily. When I pull my lagers out of the lagerator (I don't usually do a diacetyl rest, since I pitch cold), the beer is actually partially carbonated because of the co2 in the cold temperature. I think that's what you're seeing- but I'd check the SG to ensure it's done, then start the lagering. If it's not completely done, let it finish up and then lager it.