I've been having this problem for months (Safari/iPad) and only on the Mumsnet site. Today it is happening every few seconds. A Google search shows a number of threads with the same/similar problem. In my case it always goes back to the same place that I was watching.
It is nothing to do with adverts, as suggested in another previous thread. It is just very frustrating and I rarely use Mumsnet now.
Mumsnet , I HOPE YOU CAN SORT IT.