your beer is apparently carbonated. when you're adding sugar now, the sugar molecules are giving the trapped CO2 something to bond to, and it starts rapidly coming out of solution.
(my dad used to put a dash of salt in beer at Pizza Hut, because he said it was flat)
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