It happens when you click to load a page, just as the page seems to complete loading, it gets redirected to soft porn sites or this morning it's redirected me to the App Store to purchase the racing post app!
I'm on iPhone 4 using safari, but I don't think that matters. My suspicion is it is something awry with the ad server that serves up adverts for bnb. For whatever reason instead of fisher price and tesco adds it's now sending us to ewank.
I can get a screenshot next time it happens but that will just show where I end up, not how I got there as there are a number if very fast redirects that you can't quite read the URL of before you end up at the final destination.