Well, it's true for me, but not for my wife, who's in the same process. :
The only difference between she and me is that she has no gaps on her work and education history - section H of the CIT 0002. She was either studying or working throughout our time here, and I had a few months where I was just looking for work. I don't know if this is what triggered the fingerprint request, though.
I believe there's no single reason to a fingerprint request - or to any other request that might be made during the process. It can be a number of things, each case is different.
What I can tell is that, as far as I can see, the fingerprint request had little to no impact on the process speed, thankfully. So, patience, guys. As long as we check all the boxes, all we have to do is wait, and maybe add a few info on the way.