I was fighting with a dragon and then the game freeze. 688 hours play in Skyrim and I have never faced this problem before. Only difference to my game now and before is that I downloaded Dawnguard two days ago. Is this common problem caused by Dawnguard or is it something else?

It could be any multitude of factors.

If PC, mods?

Was anything particularly straining on the game happening? I.E: Many effects onscreen?

I'm running PC, no noticeable increase in crash frequency, the ones I do have can be chalked up to mods.

It happens, nothing to get all freaked out over

It can't be about mods. I currently use only Crimson Tide which I have used several months without any problems.
There was a few effects running on: Dragon coming down from the sky, my arrow ready on stretched bow, blizzard going on and my follower coming somewhere out of sight on other route because he was too clumsy to follow me on the cliffs.

Hmm.

Sometimes, the game just freezes/crashes seemingly randomly. It's perfectly normal.

And you're only running one mod?

I have fifty.

yeah it started happening to me recently too. from what i've heard it's related to dawnguard, possibly a mod conflict.

Yeah seriously lol

Had freezing/crashing/missing textures before I started using a 4gb exe (first through mod, then official).
Since then I've had maybe 2 crashes and I've got loads of mods.

Really impressed how stable Skyrim is compared to previous TES games.

I'd go more with probably. From what I know and have experienced, the PC version has virtually 0 serious issues that can not be chalked up to console command or mod usage.

Skyrim's frozen on me about two dozen times so far. I play on PS3, and I can't download anything on it (sporadic internet, no router), so I'm stuck on launch version (1.0.00.00?). I can play about 150hrs per character before lag starts.
But to be specifically on-topic, on my second character, a mage (many effects to sling around the enviroment...), instead of lag, I was experiencing random freezing. No prerequisite lag -- the game would be running perfectly until BOOM, freeze. At the worst times, too, LOL.
It actually worries me, because I feel like freezing and having to reset the console -- and in extreme cases, unplugging the power cord -- has serious, though small, risks of damaging the console.

At least it doesn't freeze as much as New Vegas.

Yes I am. I have tried several but haven't found any other which satisfies me enough to keep them running. Sometimes I make small mods for certain characters but with this one I have none of them if you don't count removing essential statuses from legates.

To each their own.

Lucky you, when i play skyrim, and opens firefox to read wikis, sometimes, skyrim freezes, my pc freezes, and on restart, it tells me to verify the HDD integrity.