Stuff like - swearing at umpires/other officials, grave swearing at opponent/crowd, throwing a chair, attacking your opponent, slapping the linesman, shooting the umpire with a gun etc.
OK, thanks Arka. I'm not too sure about that last one, surely if you shot the umpire with a gun they wouldn't be able to default you, on account of being dead/unconscious and all?
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Tim Henman famously got defaulted when he hit a ball away in anger and struck a ball girl with it. An accident, and he was totally cut up about it being the way he is, but an automatic default. I think he was playing doubles at Wimbledon.
Persistent umpire abuse/striking an official is more likely.