It will usually be an advert having a fit - a big video ad or similar, this can do it to your browser.
Can you let me know if something like this loads, and what for? We can try to block brands if we need to. They all get a bit overexcited this time of year and spend their entire ad budget. This often means heavier ads for all of us.
Ed, this only happens to AB which isn't exactly bandwidth intensive, so rather than bother to do anything about it (as it isn't my problem) I save myself the hassle and do something else..