Maybe they all wore rubber-soled shoes or practiced sneaking about; it was a silly way for grown people to act.
They might've been practicing sneaking around or pretending to be ninjas, either way, it was silly to see adults act like that. 
mnli hypothesis: They might've been practicing sneaking around or pretending to be ninjas, either way, it was silly to see adults act like that.  premise: Maybe they all wore rubber-soled shoes or practiced sneaking about; it was a silly way for grown people to act.