You are passing which is fine in that case. But there are times when drivers cruise in the middle lane despite having open lane(s) to their right which can cause traffic.

If middle lane is going at a speed of 1 and they're passed by cars at speed of 2 then the cars at speed 3 need to slow down (traffic) or merge over 2 lanes to the right to pass the middle lane on the right and then merge back to the left which of course is dangerous. Not saying I support that way of driving but if you keep an eye out for it next time it'll be very apparent.