Rockhopper is right. Because of the space restrictions on a motorcycle engine the cambelt pulleys are much smaller than most other applications and the belts are consequently wrapped around a much smaller radius when the bike is stationary. That means that the belts tend to adopt the shape of the pulleys that they are wrapped around on a bike that hasn't been used much and develop a little bumpy bit or tight spot.
As I understood it, reading it from the Sigma site or somewhere like that, the belt doesn't often break of it's own accord (although that's not to say it couldn't do that). What happens is that as this less flexible bit goes around and around (especially at 10,000 rpm) it can jump a tooth or two on the cam pulley, that puts the valve timing out of the window, piston then meets valve and the belt snaps when it can't jam the valve into the piston any harder.
£30 belts or thousands for an engine rebuild (which would write an ST2 off effectively)? As rockhoper says - your call.