He probably got caught with stacks and stacks of language/grammar porn by his wife.
"Dear, what're all these notebooks? Is THIS why you haven't been spending time with the children? THIS!"
"Uh...uh... I'm...writing a book... a book about... elves... and dragons and magic... so the elves needed a language and I couldn't just give a language to the elves and not the dwarves, ya know"
"Uh-huh, and where is this book?"
"... gimme a week, needs editing"
And that's how Tolkein shit out the Hobbit