I was walking along the tundra when a frost troll started to attack me. So as I'm fighting it, an elder dragon appears, and helps me rip the troll apart. As the troll falls I turn towards the dragon and prepare for a battle when the dragon just takes flight and leaves. I was very confused, as I have never had a dragon help me like this, and no I did not use a shout to call him. Has anybody had an experience like this one?

Lucky you....
Some dragons are friendly unless when attacked