random mortgage audit

A little info needed, we were told last week our mortgage was offered subject to valuation, valuation took place monday, were told yesterday morning valuation was satisfactory. However, i phoned bank this morning and was told offer was being processed, phoned again just now and have been told it was pulled for random audit. The guy was really good, he phoned auditors who apparently said they were finished with it and it was back to underwriters.

Can anyone explain the random audit thing? And also why does it have to go back to underwriters? what are underwriters doing now as its already been passed them?

Our vendors solicitor is getting twitchy but nothing we can do, right?

thanks im a little stressed