Why did my rod break?

This is certainly a loaded question. There are tons of factors that go into a rod breaking. While no one can determine how a rod broke without being there in person there are a few things you can make sure to not do to help prevent breakage.

Most common precautions to take:

1. Do not "boat flip" by lifting from above the rod label  - This is when you place your hands anywhere up the rod above the label section. We tend to grab further up the rod to get better leverage to lift the fish out of the water. That feeling of struggle/strain that you experience when trying to lift from the reel or handle area is similar to the struggle/strain that gets applied to the rod when you place your hand further up the rod. Your hand being higher up may help you to lift the fish up but it puts a different leverage point on the blanks natural bend/curve that it was not built to withstand.
2. Do not high stick the rod  - This is when you have any thing on the end of your line (hopefully a fish) and you hold the rod straight up in the air. Just like hand placement in boat flipping, lifting your rod straight up causes the tip to have to bend over farther causing the tip section to act as a backbone. The rod tip was meant to be a tip so try to keep your rod no higher than 45° so it can keep being a tip and the backbone can act as a backbone.
3. Do not drop your rods where they land across a hard surface  - Being mindful of where you drop your rod is definitely one that can slip our minds or habits. If you think about it, almost anything that is repeatedly dropped across a hard surface will develop a weak spot. This is even more the case if you are dropping a rod across a rock or gunnel where there is usually a bait tied on that then bounces to cause multiple times of the rod hitting that hard surface. Many people can go years of just dropping their rods across the gunnel when they unhook a fish or change rods then out of no where the rod snaps on a cast or hook set or even the next time of dropping it. It wasn't that exact moment that broke the rod, it was all of the smaller dings that took place many many times prior.