I suggest you speak to an agent in Dumont, or whatever specific town you are interested in, and they can tell you exactly how much the increase (what the percentage) has been over the past new years. It's easy for an agent to provide you with that information.
(I don't cover Dumont, or I would have looked it up for you )
What causes taxes to go up are simple little things. Gas increasing, heating oil costing more, salaries going up, needed repairs that can not be put off any longer (could add heavy to taxes), and so on.
Sometimes, in some towns you may find they have a funding crunch. They offered lavish retirement (and other benefits) and can not pay them. Those could cause a big increase in taxes. If instead the town had simply saved for that it would not be an issue.
You also have both county and state costs that are passed to the cities and towns. If the state passes on some costs to the towns taxes could jump a fair amount. Sometimes cost cutting at the state level means it increases costs at the local level.
I am from Maine, not nj, but these concepts should apply everywhere.