Either something changed or you made a mistake entering something in one year or the other. Tax law changes would have made a minor difference but no where near that much. Since we can't see your tax returns, we can't be of much help. The best thing you can do is lay your 2015 and 2016 tax return side by side and look at them line by line to see what is different. You may spot an error of discover a change you weren't aware of.
