It happens because the brain is using multiple senses to understand what's going on like how someone uses sight and hearing to decide which shape is bouba or kiki.
She says that if you were to look at the words bouba and kiki, which are totally made up, one possible explanation for the effect could be the appearance of the letters.
First, you show people — specifically, hearing folks — a lumpy shape and a jagged shape, and you tell them that, in Martian, one of those shapes is called " bouba" and the other is called " kiki."