A lot of people will just renew to save the hassle of ringing round so they like to tempt you in cheap for year one, then screw you afterwards.
That's how those "buy now, don't pay anything for 6 months" deals work. They're fine so long as you pay the balance as soon as the 6 months is up, but otherwise you get stung with draconian interest rates. Apparantly, a vast percentage of people just forget about the date, so get screwed that way.