apparently it happened to HEAPS of people at same time (Haderus)
It has nothing to do with chat spamming. The 'messages' are communication between the client and the server. Things like position updates for your character, skill activations etc. Sometimes this traffic gets stuck somewhere on the way due to an internet hiccup, and then arrives all at once, triggering the 'spam' error.
Certain addons are known to make this more likely to happen, because they increase the amount of messages being sent back and forth.
three things cause this error according to ZOS, the number one thing is add-ons running a lot of them and the transfer of data back and forth, the second a lag spike then all the data tries to catch up at once then floods in, last and most rare is someone chatting to much, a dev posted this in a reply some here on the forums I read it the other day.