I was moseying about on a mountain-y road between Solitude and Markath with my new Altmer mage, he's fairly squishy, but very powerful at only level 5 when I found a snow bear. Many of you will know that they are both fast and strong. AND RESISTANT TO FEAR! So I naturally tried to start running, I was low on health when Farkas, Athis and a few other ones show up, out of nowhere and pull a http://tvtropes.org/pmwiki/pmwiki.php/Main/BigDamnHeroes on me. Needless to say I was grateful, but their dialogue was the same as the jorvaskr dialogue, saying every new visitor in whiterun gets weaker looking, or something. I was wondering if they had been contracted to take out this bear, or it was a bug that happened in the nick of time; either way

I had a very similar event happen to me as well. I can't remember all the details, but I was getting PWNED by something and I remember thinking I was a gonner, and out of the blue the two brothers spring into action and defeat my foe and save my life! I remember being very sad I couldn't actually talk to them and tell them how grateful I was. I think it's a scripted random encounter though.

I was clearing out a Word Wall once and Vilkas and Ria showed up and helped out. I think it's a random event.