SharePoint folder name overwritten by document name

I use a SharePoint library on our company site. I didn't set it up but I am responsible for managing the folders and documents within it - the folders are named after clients, and then we store different documents related to those clients (contracts, correspondence etc) inside each folder. Today I was alerted by someone that the names of two folders in the SharePoint library had changed without anyone changing them manually. Turns out that some new documents were saved into those folders and the folder names of both were overwritten by/changed to the names of the documents.

I spoke to our SharePoint admin and he insists there is no record in the Activity log of that happening. I changed the folder names back manually but need to know what caused it so I can prevent it happening again. Anyone seen this happen before?

Edited to clarify: Most users will drag and drop documents into the folders. At most, they will 'Copy to' from other SharePoint sections and then navigate the relevant folder in this library.