I would suppose an event could have caused an eruption.
https://en.wikipedia.org/wiki/Chicxulub_crater
Considering the various ages of the ash it was a recurring event? This was likely one of them.
Or an unreliable age-testing method. If they use some crappy lash-up that inflates age in order to flack evolution and an old Earth; then it’s going to give bogus readings even if and when they’d prefer accuracy. IMHO, of course.