Well, such behavior never occurred with other games such as chess, etc.—even with card games (outside normal strategies as in poker). I'd suggest the intrinsic design and 'psychology' of Monopoly is the reason. From comments here and those I've read about over the years my experience is far from being unique. Perhaps that's why it's always been controversial and often generated stories like this.

I'm not a psychologist or behavioral expert so I'm unable to make any authoritative comment about why my family would behave differently to perhaps, say, yours.