I don't think I am equipped to diagnose what the root cause was here. It's even possible that this instance wasn't intended to have viral posts (i.e. high profile posts that get would get shared to many external users) and they didn't want to invest in hardware/services to facilitate this.
The question is whether the server was having issues with a flood of new posts being sent in and stored, or a flood of anonymous users clicking a link and blogging down when the same html was getting rendered over and over.
Knowing Mastodon, I have a bunch of was the latter with the server coming out on all the new data it was trying to store locally