I use Outlook to download all of my email from Gmail. In the last few weeks I noticed that Gmail is labeling many of my valid emails as spam. These are from sources that have sent me emails for many years. I get about 20 to 30 emails a day and about half of them are now being defined as spam by Gmail. What happened recently that this would occur? Two or three times a day I go online to Gmail and transfer the emails in the spam folder to the inbox by designating them as "Not Spam".
There's probably not a way to answer this. At least, not by us mere mortals.
The algorithms Google uses to identify possible spam are secret (and for good reason). They're also constantly adjusting them to address new threats and to prevent false positives.
More than likely, a recent adjustment to the algorithm is making the messages you receive look more "spammy". It's annoying, I agree, but the best thing you can do you're already doing: Marking such messages as "Not Spam". The algorithm should learn from that.
The only other suggestion I can make is to ensure that your mail client (Outlook) isn't also marking things as spam with its own algorithm. (There was another question on here in recent months where someone would mark messages as "Not Spam" and they would end up back in the spam folder. It turned out it was the mail client they were using, not Gmail.)