Peace first, then ...
We didn't do anything wrong
I doesn't mind what you (us) are thinking, SERP are not rational . Someone can do nothing wrong, and gain traffic one day, and loose it the next day. The same can apply to someone doing fishy things. Also, what you (us) think is done right, might be considered wrong by Search engine, for any kind of legitimate reasons, or not.
That being said, did you loose traffic for all your pages, in the same way? May be , your traffic lost concerns only a couple of pages, which, so far, were bringing good traffic. So you should review more in depth, and try to see if the loss of traffic is concentrated on some pages, or if it's really your whole site.
It's possible some of your most prolific pages have lost position, in that case, you can try to figure out who is now ranking above these pages. May be someone published similar content , Google might favor fresh / recent content. May be someone scrapped your content.
This can really be anything.
