The goal was definitely to impede the other researcher's work, and I can imagine a few possible reasons for that. In descending order of probability, interpersonal conflict (in my experience, graduate students in the same lab tend to either become best friends or hate each other, with little in-between), trying to beat the other student to the punch w.r.t publication, or good ol' schizophrenic delusions that the person's work needs to be stopped (mid 20's is a pretty standard age for onset in men).