I've had some very negative anxiety-driven experiences with dissociation (depersonalization / derealization / de-something-zation), usually when traveling to somewhere I'm unfamiliar with. It usually only lasts for 5 to 15 minutes rather than hours, but it does sound a lot like what he describes.

Relating this to meditation: On the rare occasion that I try meditating, it doesn't trigger anything negative, but I have found that I can induce such negative dissociative experiences on purpose (at least to some degree) by inducing a certain meditative-like thought pattern. It's hard to describe, but it has to do with time. It's like the mind is usually thinking at least a fraction of a second ahead, it's focused at least just a little into the future. If you break that, then meaningfulness seems to collapse and... it's not good, and you have to snap out of it.

I wonder if meditating is triggering something like that for the author.

Instead of meditating, mindfulness, thinking about your own thinking and purposefully trying to dissociate from yourself by conscious force, it may be more helpful just to find something that helps you relax. The point is to clear away stressful thoughts and thought patterns by letting the mind focus on something else rather than nothing (or itself).