At my school volunteers were recruited for course credit. So it could be due to the elation of getting it over with.
I could also argue that the effect is due to the excitement of participating in a study. If the declared interest of the students is Psychology they might simply be excited to do the study.
There may also be a novelty component where the change in routine produces arousal.
Even n=1 can give a statistically relevant result (for the hypothesis that a given event never happens)
I haven’t read the paper, but n=51 can be more than high enough to get significant results for more realistic null hypotheses, if the effect is large enough.
Having said that, effects rarely are large. If you have a weaker effect, you need more subjects (picking a somewhat random paper from a google search: https://www.journalofcognition.org/articles/10.5334/joc.72/)
I’m surprised by the fraction of male subjects they managed to recruit. A well-known joke is that the typical experiment in psychology is either done with mice or with teenage girls as participants.