A bot, or bots, scraping the site. Making 10,000 connections as fast as it can.

[Edit] MedicineStorm beat me to a response. :P

This was something briefly disscused on the OGA Discord server and as MedicineStorm said, it's a Bot scraping the site with thousands of connections.
I did notice the site crawling along yesterday from around 10PM GMT, seems to be back to normal now.

If you see it again, do not hesitate to @ me on discord (or IRC/matrix, but that doesn't give me an alert, so it may take me a few more minutes if you're notifying me via those channels.)

It may be a good idea to make a note of our discord/IRC/Matrix server now, while the site is running fine. It's hard to find that front page "chat with us" link with a 502 error in the way. :P

Discord: OpenGameArt
https://discord.gg/yDaQ4NcCux

IRC: #OpenGameArt on https://freegamedev.net/irc/#opengameart