As title says:
AH1 will begin to slide as soon as pilot has boarded and game physics switches to Advanced Flight Model (usually one second after boarding animation complete)
With the engine off, this slide cannot be countered and the aircraft drifts until it touches something, then naturally, explodes.
If engine start is performed quickly enough to prevent disaster, it is then possible to control the slide by moving the cyclic back and forth. One may even trim it out to a position of balance, but any nudge on the controls will cause the sliding to begin again.
Needless to say, this makes refueling and rearming the AH1 a rather awkward process. In MP, it can easily be the cause of much embarrassment.