I just must be hitting it, or the Alt key at the wrong moment. I have Googled this and I am certainly not alone in having the frustration: but the only answer I can find is that the Ctrl key must be stuck. The frustration is that as I type I suddenly find that some window has opened up or my cursor has shot to some other part of the page and the last sentence has come out in a different paragraph.
In fact, none of me is what it what once was, but we don't need to go down that road. I am frustrated by the fact that, although I type quite fast, I tend to nudge other keys at the same time.