negative pressure creates a vaccum that sucks it back in. but it should not keep doing this unless your lager fridge is experiencing wide temp ranges.
Also, you should really put NO-RINSE sanitizer in the airlock, or vodka. Water won't kill any bugs that get in like 40% ABV vodka, or some star san solution.
don't fill your airlock with bleach water!
Primary: English Mild
On tap: Pale Ale, Lancelot's Wheat, English Brown Ale, Steam Beer, HoovNuts IPA
Bottled: MOAM, Braggot, Raspberry Melomel, Merlot, Apfelwein, Pyment, Sweet mead, Cabernet
Gal in 2009: 27, Gal in 2010: 34, Gal in 2011: 13, Gal in 2012: 10