Customer from my current employer offered me a job and I'm taking it. How do I resign?

Title says it. I met this person at my current place of employment. As of yesterday, I've formally accepted a job offer from their company. I'm unsure how to explain what I'm doing or where I'm going. I know I have no obligation to do so, but this is really out of left-field because I truly wasn't looking for a new job. They're going to be quite confused, and they're great people, I can't/don't want to strong arm them into being okay with not having any answers or explanations. It just feels weird to say "A customer of ours that I met on your dime offered me a job and I'm taking it." It doesn't exactly roll off the tongue. Any advice?

Also, the job I'm taking is not even remotely close to the same line of work I'm in now. I'm not being poached by a competitor.

Edit: Seems like most would agree that I shouldn't mention anything about it. I never like this approach, but I suppose it's the best way. Thanks for your input!