Maybe it has something to do with maintaining compliance with the IRS - or at least being able to say that on a certain schedule, they verify account information. Or does that make no sense whatsoever?
More likely they are weeding out multiple accounts held by one person in an attempt to hold down the scammers.
People likely to get caught in the net of this are people who share a same name and same bank with another turker, couples who have separate turk accounts but share a payments account, inactive users, or users whose banks have changed names over the years and have different routing info. A lot of businesses do routine re-verification but they usually don't put a notice up on their site. I know paypal randomly redoes this every few years.
Now if a turk account is put on hold it's probably because the payment verification lead to something that looked suspicious or there was a simple error with the re-verification. In a perfect company it would be easily fixable once you contact them. We all know, however, that Amazon is not the perfect company.