I am just guessing, but it sounds as though they are just trying to connect TH to any criminal-type behavior in the past? Possibly at the suggestion of a profiler. To determine is she is in an escalating pattern of some sort?
But going back 20 years, that is a stretch if in fact she has not been steadily committing increasingly violent and anti-social crimes since then. Arson is often a "starter" crime, but usually the offender moves on and continues to progress. I have not heard of too many offenders who can go 20 years without being caught doing something, if they are continually offending.
Anyway, I think it came from a profiler suggestion.