Generally, at least for riders who are still learning wheelies rather than more experienced riders who are going for the 12 O'clock, if you watch video footage of 'em you'll usually see the initial launch has taken the front of the bike a lot higher than they're happy with so they close the throttle [1]. They then realise it ain't as high as they thought and that they can keep the wheel up by opening the throttle again. Except, of course, the engine's higher in its rev range, and therefore more responsive, which initiates an even more vicious rising of the front end which is generally terminal - and happens in a split second, too

The other cause is riders who let the bike control them rather than the other way round. They launch the front up but don't have the experience to know a certain throttle opening equates to a certain height - so they launch the wheelie and just keep the throttle open, except it's too open and the bike just goes straight over

If you're learning to wheelie then whatever technique you use to get the front up, never worry about gaining height - start small and work on being smooth and on holding a nice smooth constant throttle. The height will come naturally

Oh, and sooner or later you will crash - it's the law


[1] When you listen to some riders popping wheelies, you can hear the throttle going 'brap brap brap' as they constantly close and open the throttle [2]. We call this the 'sound of fear' as it's their subconcious forcing their right hand to close the throttle (standard self preservation reaction) and their concious brain telling the subconcious to **** off 'cos it wants the front up

[2] On a full-on 'crosser you have to 'brap' the throttle, on anything else it's simply bad technique

 

Clutching it up seems a lot smoother than powering it up.. Keep your foot on the rear brake though when clutching it cause the motor is a lot less likely to bring it back down after hitting 12... If you power it up the motor will bring it down when you let off the gas after hitting 12... I think clutching wheelies are way better...