I've had this happen to me a few times, where I'm browsing a website or using an app, and all of a sudden the app store launches - or an iTunes page pops up (seemingly randomly).
I discovered that the reason behind this was "hidden links" embedded in the content of the site/app.
Sometimes these links are directed to pages associated with the app/site I'm using, other times I suspect they might be "paid links" (as in they are getting paid to direct traffic to that page).
Either way, it's a scammy thing to do!
If this happens to you, then I'd report the app to Apple from the following page: http://support.apple.com/en-us/ht1933
Just don't get upset with the support team if they don't have all the answers though!