A driver coming up behind you as you're starting to set off ( or immediately after you've fallen down) when the queue ahead of you is already moving. Somehow you tend to blend into the queue visually - the driver doesn't see you as separate from the vehicle in front of you, and doesn't realise you're still there when the vehicle starts moving (or does realise you're there, but rushes past you unsafely) - and the risk of falling over is much higher as you're starting off than at any other point.