I highly doubt that would be enough to break that rod. I suggest you take it back, explain the situation. I’m betting the store will replace it. If not, contact the manufacturer. There is a warranty, just more hassle for you.
As for why, the rod was likely damaged somewhere in it’s life. I watched customers beat the snot out of rods when I worked at Gander. I watched employees break rods. Some people just don’t get that graphite is brittle. The rod simply falling against the edge of the boat could lead to a break in the future.
Good luck getting it replaced. Get another IM8, I find it unlikely you’ll have the same problem. 