Killed just about everything in my wake, including essential characters via turning them into ash piles through Raise Zombie spell. Since I have kill-able kids mod I also took the liberty of killing all the kids that were annoying to me and turned them into exploding thralls. Twas worth it.

the dlc is a godsend to alchemists though :)

Three, you forgot to count Hearthfire.

I've thought another reason, it's the will of the legendary immortal hero. Since in all the game it's hinted through dialogue and various lore and references primarily in Before the Ages of Man last sentence. And in a way you are considered the immortal legendary hero, thus strange phenomenons always follow you, strange events always following you, traps resetting, food that were rotten coming becoming fresh the day they were stored, etc.

perhaps there were previous adventurers and bandits that were using those barrows and happen to get in through an unmarked entrance and are capable of evading all draugr spiders traps.