Last minute odds- stupo question from newbie

Greetings.
Forgive a possibly silly question, but I've only been using Betfair since last week, but with some success so far on the lay side of things. My question is- why do the prices go berzerk in the last couple of minutes on bet/lay before the off? One of my selections I laid at 22 went up to 1000 on lay and 150 and upwards on back. Is this just the mathematics settling down, or is there some benefit to this?
Thanks in advance for you input.