1. Numerous reasons, I'd suspect. The games are filled with instances of risen dead like the skeletons often seen around graveyards and forts in Skyrim and Oblivion.

2. Andur's quest suggests so, or at the very least Arkay's blessings prevent necromancy and but not the dead rising on their own accord.

3. Yes. Play any other game and you see it occurring.

The Skaal attribute the All-Makers curse for acts of cannibalism as the source of Draugr on Solsthiem. Just another example cursed undeath

Why are they considered such a mystery then? It seems like the draugr would then be easily dismissed as creatures who haven't received Arkay's rites. It would be easy enough to dispatch a few priests to a tomb to put things to rest, maybe create roving order just for the purpose. The skeletons are interesting considerations, too. In most cases they just seem to be the subject of necromancy. Perhaps some of the skeletons in Skyrim are just draugr that have fully decayed. But having the dead rise of their own accord is a very specific cause. I haven't played the other games, so I wouldn't know if reasons are ever offered, other than necromancy.

Ah, that's an interesting inclusion. Although with Solstheim also home to Dragon Priests, the actual cause of the curse could be different.

Definitely. As much as I love the Skaal, I don't know what to make of their All-Maker, or how much credit can be given their beliefs. I'm of the opinion they interpret things a little oddly due their religious views, so if there is truth behind that legend, it may be lost in translation and interpretation.

I feel like they must be doing something right, since the All-Maker stones do respond to them, but maybe they are just natural magical batteries or somethin'.

Are they a mystery? Draugr seem to be well known through Skyrim, and folks warn you to be careful around the ancient barrows because of the Draugr.

Skyrim, you have to remember, is very old and has had countless battles fought there. It's one of the oldest provinces inhabited by men, and it is littered with tombs, barrows, and crypts. Entire cities have been lost in the mountains and snow over time.

I'd wager there are simply too many tombs, too many draugr, and too little danger from them to warrant such a massive project of constantly patrolling to make sure the dead stay dead. Most crypts have been built specifically to keep the dead contained inside, as if they knew they'd eventually get up and wander about.

They certainly seem to have validated their beliefs time and again.

The All-Maker could be an amalgamation of the natural forces of the world. Arkay and Tuwhacca could be merely recognized as localized cultural "ontological avatars" for a greater empyrean force governing life and death. There are other realms of influence, recognized by other cultures as having stewardship by separate entities, but the Skaal make no distinction, seeing all forces as part of the All-Maker. 

They would still be capable of interacting with the worlds forces even if they didn't bother to distinguish them through name. 

Didn't mean to turn the topic aside. Lol. I'm glad you okayed tangents in your epilogue. 

I'm not saying the draugr are an unknown, I'm saying that what causes them is. From the loading screen:

No one is quite sure why the Draugr walk the halls of Skyrim's burial crypts, but some believe they once served the dragons ages ago... and have been cursed with undeath for their treachery.

The Gauldursons do make an interesting case for crypts built to hold the dead, though.

No one does know. We do know necromancy isn't a requirement. The dead can just get up on their own.