What causes Christopher to wet his pants?

From the text:

And then I wanted to go for a wee, but I was on a train. And I didn’t know how long it would take us to get to London and I felt a panic starting, and I started to tap a rhythm on the glass with my knuckles to help me wait and not think about wanting to go for a wee, and I looked at my watch and I waited for 17 minutes, but when I want to go for a wee I have to go really quickly, which is why I like to be at home or at school and I always go for a wee before I get on the bus, which is
why I leaked a bit and wet my trousers.