I think the main problem here is the difference of interpretation between "overtaking" and "passing." Overtaking is an active process, involving signalling, moving out (to the right) when safe and then moving back into the same lane. Passing, say on the left lane can mean the driver does nothing in terms of speed - merely that the traffic lane(s) to the right have slowed for whatever reason. This happens all the time especially on the likes of the M50 - even at relatively high speeds. Quite typically it can happen when someone on the outside (fast) lane moves to the center lane often into a narrow gap. Drivers then hit the brakes momentarily and that lane slows down. In practice that shouldn't mean everyone in the left lane should follow suit - for no reason. This is how pile-ups can happen...