What is the cause of this short in the outlet receptacle?
The most likely explanation is that the screws were loose causing a poor connection, and arcing under load - evidenced by the fact that the base material has gone brown where it's been hot over a long period.
What happens in this case is that the contact surfaces slowly carbonize (so become higher resistance, and get hotter), and it gets progressively worse
You can see that the right-hand screw-terminal has actually moved because the plastic melted. I assume that it moved enough for the contacts inside to either come into contact or arc between themselves, and that's what tripped your breaker.
It's unlikely this will repeat itself, provided the new outlet has been wired correctly, but it wouldn't hurt to keep an eye on it for a few weeks, just to be sure. Also, as mentioned in the comments above, check the plug and replace if there's any evidence of damage.