Well first of all, I'm sorry to hear about the bad beat. It must be a terrible sight when something like that happens. I have seen a bit worse than that (15v1 losing) but its still pretty bad.
As for the comment, "how is that possible?"....well, the rule of large numbers my friend!
Let's assume that you lost every attack, so your defender still had 2 men left at the end. The probability of that happening would be 4.6*10^-4%. Not very good of course, though the actual odds of losing the attack outright are a little higher because you could also defeat one of his men but then lose to the rest.
Anyway, Andy has posted that CC goes through 125,000 lines of dice rolls per day, though that number was from november. Of course we must go through even more than that now because the site is ever growing, but how much more I can't say....so I'll use 125,000.
The math from there is simple- 4.6*10^-4*125000 = 57
That means that if every attack made on the site was 3v2 dice, this would occur 57 times a day. Of course people have many different types of attacks as well, and they won't often be in the 20v2 situation you described. But if they were, despite the small odds of it happening, it would happen multiple times a day simply because so many dice are thrown around this site.
It's just a shame that it had to be you on that occasion!
Edit- Sorry, I misread your post based on the title. You lost 20 men and defeated two, not lost 20 men and defeated 0 because you were facing 2. Well, that just makes the chances of it happening a bit higher!
*Awaits maniac to show up and make corrections...*