Is that pending balance or actual balance? I think I read a few threads back something about exchange delays. So it could be an exchange returning BTC with a low fee for the current competitive market place for block space, so taking a while to confirm. Or it could be not enough coins are being sent to that exchange to trigger the minimum withdraw balance (which keeps BTC withdrawal fee % low), or something is just a bit craptsy.
If its pending balance, that can get a bit chunky if mining some of the newer algos which have long block times. A bit of luck with good hash rate on algo solving a high value block can make the pending balance jump, followed by a slower period where blocks are not found, or only low value blocks are found.