There are two "respectable" theories that I know of - the whale oil theory, and the "soft coating" theory.
I do not believe in the soft coating theory for the following reason: I have seen lenses with lens haze from both the period of (internally) soft coated lenses and from the period of (also internally) hard coated lenses. To be more specific, all Summarit 1.5/50mm lenses are safe to be assumed as internally soft coated lenses, the same goes for Hektor 4.5/135mm lenses, but all lenses above SN 2,000,000 are safe to be assumed as internally hard coated lenses, and I have seen hazy ones of those lenses, too, e.g. Summicron 35mm (8-element) and 50mm (7-element) lenses with nos. well beyond 2,000,000.
The big difference is that the latter lenses may be cleaned without taking off the internal coatings, while the former will lose their internal coatings in the cleaning process and will have to be recoated.
In my view, the lubricant evaporating theory and the microcondensation theory are both plausible and will likely both contribute to the hazing effect in varying degrees, depending on user circumstances and geographical location of the lenses.