I'm pretty sure I know the answer - Mint uses a back-end communication technology (previously Yodlee, changed after the Intuit purchase) to communicate with your financial institutions. It seems highly likely that if the Mint <--> Institution information isn't passed along in ~15-20 seconds or so, Mint will throw the default 'attention/communication issue'.
The back-end communication technology is what uses your login credentials to access your account, scrape the various text, assemble and search for relevant data, then present it to the Mint.com front-end. Some of the most common examples of this in my various accounts are small-time local banks. I'm betting it's likely that their account presentation pages are slower to render, and cause the occasional issue. Re-updating or occasionally asking it to Fix the account usually solves it nearly immediately in almost 100% of cases.