It's primarily a matching failure between what you see and what your inner ear, proprioception, and sense of pressure are telling you is happening, as well as any other visual input if it's a mixed-vision system. Mostly inner ear, and vision if present.
Also it's mostly rotational motion that triggers it - but you'll easily hit that with just the small motions of your head and the looseness of the headset.
Which is ridiculously hard to compensate for since you have to get extremely precise, low-noise motion & environmental pose tracking, with extremely tight time limits - and then it has to be fed into the rendering engine, which has to react in time to fool the operator.
But the evolutionary rationale is also right. Your brain evolved to freak out in the event of persistent sensory mis-match because it's a pro-survival feature. The naturally occurring causes are things like severe food poisoning.
> I thought that I was going to have problems as a user of reading glasses, but that turned out to not be a problem.
IIRC, the reason is that VR stimuli are "focused at infinity." It also helps that you're trying to take in the image, not the pixels or other fine details.
Trying to read small text at a distance of ~ 20 cm requires relatively precise focus. A picture of a cat that's taking up your entire field of view does not.