Iphone Moving email from Inbox to Junk Automatically

I don't delete email unless it's obviously spam because I never know when I might need it again. Recently I noticed that emails were just randomly disappearing from my inbox. I'd try to find the email confirmation for tickets I had purchased, or one from a coworker with details of next week's meeting, and they were just gone. Months and months of emails missing across multiple inboxes.

I FINALLY discovered that these messages were getting moved from my inbox to the "junk" folder on my phone. But I didn't do it.The junk filter also appears to have gone into overdrive because I see new messages in there that I never saw in my inbox and are definitely NOT junk, and have never been filtered before this started happening within the last week or so. I have both yahoo and gmail accounts connected to my phone, and it's happening with all of them, so it's not host-specific.

Why is this happening and how can I make it stop??