spam inbox question

This is a bigger question than it sounds like, unfortunately. First, how are you seeing this? Are you using a third-party inbox monitoring tool, or your own seed list, or something else?

Second, is it limited to one specific mailbox provider like Gmail, Hotmail, iCloud, etc., or is it across all mailbox providers?

Third, has anything changed lately with your sending, like your content? Or has your audience unexpectedly grown? When was the last time you sunset unengaged subscribers, and is it time to do that again?

Mail starts going to the spam folder when the mailbox provider sees indications that the mail is unwanted. If people leave the mail in the spam folder, then the mailbox provider has confirmation that their assumption was correct, and the mail is spam, so more of it goes to the spam folder and eventually may be rejected outright. If people move the mail from the spam folder to the inbox, or if positive engagement increases (like opens, clicks, replies, read-time), then the mailbox provider has input that indicates perhaps the mail should actually be sent to the inbox for more people, instead.

In the short term you'll want to narrow your audiences to focus on your most-engaged subscribers, and wait to see your results increase. The positive engagement from your biggest fans will help rebuild your reputation, and you can eventually begin folding back in the unengaged folks. BUT, if you haven't solved the root issue of their lack of engagement, you'll just be in the same spot again.