Loosehead, as I read it, Frazer is referring (for example) to a dual carriageway (with 2 lanes of traffic in each direction), where some bikers drive roughly along the line separating the two lanes in one particular direction. As they encounter cars in either lane, they move slightly to the left or right to go round them. This can be fine if there are long gaps between the vehicles but can cause problems where vehicles are close together.
For example, Frazer could be driving a car in the nearside lane when a biker (who is basically following the line between the lanes) comes up behind him and moves out slightly to pass Frazer. So far, so good. But immediately ahead of the motorcyclist is a car in the right-hand lane. The motorcyclist now pulls over to the left to get past this car (on the inside). Because he starts this manouevre almost as soon as he's past Frazer, he cuts in front of Frazer, causing him to brake and/or swerve.
As soon as the biker has passed that second car, he's now got another car in the left lane ahead of him, so he's moves slightly to the right and into the path of car number two, causing that driver to brake and/or swerve . . . . and so on.
As I've said, I think that this is the type of situation which Frazer is referring to but he'll probably post again to say I've got it all wrong anyway ;-)
Chris