Maybe you could use a combination of the two different strategies suggested here. You're taking the new job because of how well it suits your needs, but you were probably LOOKING because of issues where you are. So maybe this:
"I was hired to do specific things, three different areas in total. A month after I was there, the job description changed completely. This prompted me to search for something more suited to my talents and passions. I enjoyed working with you all, but I have to take advantage of the opportunity being offered to me."
It's honest but not accusatory. It can't come as a shock to them that changing job descriptions on people is not a good thing, and you won't look spiteful for pointing out something that had to be obvious. This gives them an honest but very discreet answer, and you've given them what they requested.