I'm sure lots of people have run into this, but I randomly came across Alduin raising a dragon just southwest from Riverwood. On the west end of Lake Klinarth. I wasn't sure it was him unitl I heard him talking. I tried firing a few arrows at him, to no avail... eventually he left, but the summoning wasn't finished. I waited around for a dragon to pop out, but nothing was happening so I left...
Here's my question: Did I stumble ahead of myself in the main quest, or does Alduin actually go around Skyrim raising dragons in "real-time"?

Yes, he does go around raising all the dragons.