29-10-16, 02:08 PM
It may well be that the plugs just got a bit sooty, as Tommy suggests. The sparks track along the insulator or across a whisker for a few revolutions until that path is burned away and the spark can jump the gap. In the meantime, unignited mixture has been getting pumped into the exhaust system so the hot exhaust from the first successful firing detonates it.
If you've had a week of short, slow commuting in heavy traffic, look on it as the bike's way of asking for a long hard run. :b