Anomally With Es Explorer

Today, I received a notification from my phone's antivirus stating that ES Explorer was exhibiting suspicious activity. I checked the app's details and found that the file manager was taking up 2 GB of storage (my current file manager uses only 30 MB).

I navigated into ES Explorer with the Android folder and found an archive inside the tmp folder. Inside that archive were... archives containing pornography I had downloaded about a year ago. I decided to move some of these porn archives from the tmp folder to a more appropriate location.

After moving some of the files and returning to the tmp folder, I discovered several more archives there, ranging from 300 MB to 800 MB in size. I spent about a minute trying to access the archives when suddenly they all vanished, leaving the folder empty. Thoroughly alarmed, I disconnected from the internet and uninstalled ES Explorer along with all its associated files.

What could this have been? A hack? Or an app bug?