Their tax bill is rising 10% a year, depending on their property tax (if it is budget based or not), this could just be that their property is appreciating 10% faster than everyone else’s property. That means their equity is shooting through the roof. If it isn’t on a budget system, that could mean that their property (and possibly everyone else’s) property is appreciating 10% a year, or maybe they are adding levies or increasing the rate to fund new spending.