The canon answer is supposedly covered, in full, on Enterprise.
However, I have only seen a handful of Enterprise episodes. Mrs. Midnight are getting to the point where we lost track of Voyager - Season 4 or thereabouts. After that - all we'll have left is Enterprise.
Laddie, don't you think you should... rephrase that?
MCCOY: Spock, I don't know too much about these little tribbles yet, but there's one thing that I have discovered.
SPOCK: What is that, Doctor?
MCCOY: I like them better than I like you.
SPOCK: They do have one redeeming characteristic.
MCCOY: What's that?
SPOCK: They do not talk too much. If you'll excuse me, sir
SPOCK: Surely you must have realised what would happen if you removed the tribbles from their predator-filled environment into an environment where their natural multiplicative proclivities would have no restraining factors.
JONES: Of course. What did you say?
SCOTT: Well, Captain, the Klingons called you, uh a tin-plated, overbearing, swaggering dictator with delusions of godhood.
KIRK: Is that all?
SCOTT: No, sir. They also compared you with a Denebian slime devil.
KIRK: I see.
SCOTT: And then they said you
KIRK: I get the picture, Scotty.
SCOTT: Yes, sir.
KIRK: After they said all this, that's when you hit the Klingons?
SCOTT: No, sir.
SCOTT: No, I didn't. You told us to avoid trouble.
KIRK: Oh, yes.
SCOTT: And I didn't see that it was worth fighting about. After all, we're big enough to take a few insults. Aren't we?
KIRK: What was it they said that started the fight?
SCOTT: They called the Enterprise a garbage scow, sir.
KIRK: I see. And that's when you hit the Klingons?
SCOTT: Yes, sir.
KIRK: You hit the Klingons because they insulted the Enterprise, not because they
SCOTT: Well, sir, this was a matter of pride.
Just a fun episode all the way around.
You bring this network's ratings down, Flavius, and we'll do a special on you!
This was also a good McCoy/Spock episode. Some funny moments, like the bickering during the gladiatorial combat, to a pretty intense confrontation in the jail cell.
One of the best Star Trek eps of all time, any series