Why are browsers makeing PUT requests for static assets on my site?

Our site hosts static assets at /assets/…. In debugging a font-related issue, I looked through our logs for unusual activity. I found a bunch of requests like these

method path                         referer PUT     /assets/js/40-8b8c.chunk.js https://mysite.com PUT     /assets/fonts/antique.woff2 https://mysite.com/assets/css/mobile-ef45.chunk.css 

The requests come from lots of different IP addresses all over the world. I don’t see any pattern in the User-Agents. The only HTTP methods are HEAD (odd, but fine), GET (expected), and PUT (very suspicious).

I haven’t been able to identify any code in our system that would cause a browser to make PUT requests to these paths.

I have no evidence that this activity is malicious. It could certainly be a broken browser plugin.

Has anyone seen this sort of behavior?