Well, VESTS are just a representation of your ownership over the total_vesting_fund_steem, so when the vesting fund takes a massive hit (from a large powerdown), it's bound to affect the overall value of each VEST in some way. You'd think it would be proportional and wouldn't impact it - but maybe there's something strange happening here.

I don't have a great answer, but due to the fact that it seems to be once a week (which correlates to a power down), it leads me to believe they're somehow related. Either that or someone is manually doing something else (powering up?) once a week like clockwork :)