Negative ranking effect when updating an app?

[SiVola]

Hello,
our app has been in the top 10 social networking for quite some time (12 months total, with some up and down, but mostly in top 25 for the whole period). Recently we have noticed that, even if the app has been stable for weeks in the top 10 positions, each update made us lose 4-5 position in just one day: from 2 to 5-6 the first time, now even 14 to 22 in one day only. We noticed this only recently, never before...

Any explanation? Did this happen to someone else? Is there maybe a bug in the ranking system that zero's the download counters for the day of the update, so many positions are obviously lost?
Thanks!