salinity changes can do that, as can additives. if you also happened to put in some melafix, or some calcium or some other of the myriad of additives at the same time as the water top off (common since we usually tend to do all our maintanence at once) that could have had the same effect.
if your skimmer has a water flow control, ie a ballvalve beween your pump and skimmer, try backing off a tad on the water so that there is more time for the bubbles to move to the top of the skimmer chamber without being swept out.
HTH