There have been a few big changes to the university system in the UK in previous years- firstly, the fees rise, which has caused applicant numbers to fall a bit, secondly, the change in student quotas which creamteas has explained, and thirdly the introduction of adjustment (where students with better grades than their offers can "upgrade" to a different course). Although adjustment was introduced a few years ago, last year, the number of students using it almost doubled.
In general, universities offer places based on data from previous years. I am guessing that this year their data didn't quite match up with what happened for various reasons, and so more universities had more space, and didn't want to miss out on the good students going through clearing and adjustment.
So, this year, I would speculate the very top universities (e.g. Warwick, Imperial, LSE) made more offers (due to lack of a cap on ABB+ students), meaning less places were firmed at the next level of universities (the Manchester, Liverpool, Leeds sort of level). Also, more universities would be keen to take students who missed their place by 1 grade so long as they remained above ABB, when in the past they would have had to reject them, so less students are using insurance places. Finally, as the number of students getting very top grades went down slightly this year, more students than usual would have missed their offers.
Also, just because a course is in clearing doesn't mean it had lots of spaces. In the past, if you were a couple of students short of the quota, I believe the government funding you got wasn't affected. Now, most of the funding comes direct from the students through fees, so if you're three students short that's £81,000 the faculty have lost over three years, which is a fairly considerable amount of money. That means there's a bit more pressure for the admissions team to fill all the places.