Maybe I'm just seeing a correlation where there is just coincidence, but it seems that whenever my characters try to read spellbooks, they get interrupted by a newly generated monster (actually, it also happens when BUC testing at altars a/o trying on multiple armor items). I understand that these actions take time, but when I've wandered empty levels for very long periods without generation, I sometimes disrobe or just load/unload my containers and sure enough... Is there a trigger, or some sort of chance enhancement really happening? Or, am I just imagining things?
You're imagining things. The generation would happen roughly the same in the same amount of time doing other things. (Technically, any action you take that uses random numbers from the RNG does influence the probabilities in future RNG-influenced things, including monster generation, but it won't skew towards making monsters more likely unless you're manipulating the RNG on purpose.)