Yep, this very common when you take control of one of one of the Roman factions. It happens mainly when your conquests cuts-off the other two families from their usual targets, because of the ways the AI uses to find its next target. These ways include (among other things, like line-of-sight etc) starting relationships betweeen factions and region proximity.
So, if you take first the regions they have targeted and intend to conquer, the AI gets trapped in an endless priority struggle, trying at the same time not to attack you (due to the roman-works-together thingy, which breaks only when the civil war begins) but also take the provinces you currently hold (due to targeting).
So, while doing this perpetual thing, the AI builds and builds armies that doesn't use, usually because it can see no path connecting its cities with their new targets (if any) or because the AI very rarely uses naval invasions. The latter is evident in the case of the Scipii, where many times (not always) they amass huge forces near Capua but never-ever even board a single unit to take it to Carthage.
This whole roman-chickenship thing results in turn in factions with expansion prespectives getting overly strong, since there are no Romans to stop them, as it usually happens. So Gauls, Greeks, Carthaginians etc become quite powerful being unchecked by Romans and you have this very unusual and interesting development... :)