Taxes are being criticized for being regressive. A scenario for this property tax criticism can be stemmed from this: property tax of lower income households being higher than those of higher income households, with respect to their income percentage. From this, we can deduce that if the activity of being taxed is less likely to be performed out by the rich is more likely to be achieved by the poor then, the tax may be considered regressive.
To add, regressive taxing can be done on different scales, such as individual taxes, or the tax system as a whole; a year, multi-year, or lifetime.