What would cause this ?

[MontyVeda]

It's eaten the hamster.

 

[midlife]

 

[boydj]

Lots of tubes will do that. That's why you don't put much air in them outside the tyre.

 

[Torquewrench]

Is it a Cobra tube?

indeed looks like a snake which has swallowed a rodent
(sorry for off-topic but couldn't resist )