He botched the spell because he couldn't maintain his concentration while someone was talking to him. You call it an interrupt, I call it a skill roll made at disadvantage (or maybe just a bad skill roll).
Either way, he messed up the spell and it had consequences.
And just generally, he's had to practice magic to get good at it, which suggests that there is a skill component to doing it.
The whole Harry Potter mythology is based on the idea that magic-capable people need to be taught how to do magic. Magic fails often and spectacularly.
You can also look at most any work involving any kind of witchcraft. The botched magic ritual with unintended consequences is so common it's a trope.