Sustained high revs can build up considerable crankcase pressure, which is what has probably caused this leak. Singles and many twins tend to suffer from this more than IL4s (which have a more equal amount of pressure being created as two pistons rise and fall as opposites).
Mine broke the breather reed valve early in the piece (after a long day on the throttle..) so take off the black plastic housing and check the reeds.
It may just be due to the (flattened) hose being full of oil with the sustained revs, and be leaking past the clamp.
Make sure you haven't overfilled the crankcase level, as this could also cause this type of issue.