Why does half of my inbox suddenly move to SPAM?

[Eddy]

In the last few days, I had it twice that when I log into my inbox, half of my emails have disappeared. I then checked in my SPAM folder and they do appear there. Strangely amongst them are several with senders and on topics that I use a lot and are certainly not Spam. Some of these had follow-up emails of a later date and had not moved to Spam...
When this first happened over the weekend, I just moved them back using "not spam" and hoped that would be it. But this evening I had exactly the same problem with exactly the same emails.
Anyone any idea what could have gone wrong?