Property tax rates don't change very much, if at all. However, when your local government needs to increase its revenue from real estate property taxes it has another trick: It just changes the assessment value of your property. When you establish that a property is worth more than it was before, you can keep using the same tax rate since it will yield higher revenue when multiplied by the more valuable property.
Other factors can increase your property taxes, too. For instance, real estate values generally increase with time. So property tax assessors reevaluate the worth of local real estate every couple of years. If the worth of your property goes up, your taxes do, too. If real estate values increase too rapidly, the government might adjust its assessment or tax rate so that residents don't get gouged. Of course, if real estate value decreases, the opposite effect would occur and real estate property taxes would drop. But if local governments can't afford to lose the income that would coincide with lower property values, they can raise their assessment rates or tax rate.
There are some other reasons your property tax bill might drop. For example, if a number of new businesses open in town, they'll bring in more money. The same goes for new construction on previously vacant lots; the raised value of the properties will generate more taxes and spread the tax burden across more properties, thereby lowering the overall taxation rate. And since property taxes go toward your local government's budget, a change in the budget could cause a change in your tax bill. A larger budget would raise your bill, and a smaller budget would lower it. Meanwhile, if the sales tax revenue in town decreases due to a bad economy or less tourism, your property tax might be increased to make up the difference.