Airlock question

Ok, so I brewed my second batch of allgrain yesterday- 8gal. of an Irish Red, and everything went like it was supposed to.  I cooled it to 70 deg. before pitching the yeast, and the carbuoys are in a dark corner of my garage which stays between 62 and 68 deg. at this time of year.  I split the batch evenly and put 4gal in each of two 5gal glass carbuoys.  I figure with that amount of extra space in the carbuoys it would be allright to just put airlocks in each.  I go to check on them this morning and I see what appears to be a (for lack of a better description) a vacuum being set up in the airlocks/carbuoys (the water "barrier" being sucked back up into the airlock).  Any thoughts on what's going on? 

The temp change caused the vacuum: as it cooled it shrank.  Recommend you break seal to relieve vacuum and replace water with vodka or StanSan, in case it repeats.

I ferment 4 gals in 5 gal carboys all the time, and have now begun to use a blow-off tube all the time.  Don't always "have to" but it eliminates this problem as well. 

I'd recommend using the 'S' type airlock if you aren't already.  I put 100proof Orloff in my airlocks.

Thanks for the replies.  I did indeed switch to blow off tubes and everything is going well now.