I feel like saying something spooky like, "If you leave the front door open, you never know who will walk in", but the fact is that meditation is just a practice that emphasizes and strengthens certain brain states/EEG rhythms, neural synchronies, and given heterogeneity in brain structure, connectivity, and genomes, there will be no guarantee that this will be an entirely positive shift in neural dynamics. For analogy, for many people keeping themselves busy at work keeps themselves from constant rumination of a traumatic past.