I’ve have similar results in the past. I had one account that was 1.2m monthly drop to 500k and is now back up to 800k. It also lost almost 4k followers overnight. I did follow/unfollow initially on the account and then quit after it took off on it’s own. I’ve never bought any followers. It grew organically from that start. I repost others content on a ratio of 8 out of 10 others posts. I assume a number of bots followed at some point, and Pinterest purged them out one night.
It could be a number reasons, but it’s usually engagement. Pinterest like other social sites are all about engagement. If one profile get’s more repins, and repins from larger accounts it helps the authority and reach of the account. Comments on a pin can make a difference also.
It could be the SEO of the pins, or keyword volume. The amount of pins and repins. No two accounts will behave identically. Keep posting quality content and repost high quality content and you should see if rise again.