I have noticed this a lot lately. In fact it has happened to me four five twelve times while typing this thread title and message.
Basically I put focus on a field and start typing. Then I look up and discover that much of what I have typed has not appeared because something has randomly caused the focus to move away from the input field.
I tried watching really carefully while typing this and it is not something I am doing it just seems to happen randomly. It might coincide with when one of the adverts is refreshing.
So frustrating!