Could have been anything.
If you read the article, there is no doubt the pilot had never been a professional, but a biz owner that somehow found the hours, and time, and someway past the insurance guy to get into her own jet.
So right off the bat we aren't talking about a 10,000 hr seasoned captain here. Read my posts on the cable airport crash...I helped train that guy...another biz owner that thinks he's a pilot.
We don't know if it's icing and I can tell you that icing didn't bring the Citation down...even so, with both engines flamed out, so what...she didn't glide to the ground, but lost control...and Citations have great deicing ability.
This is pilot error, or possibly there was just something wrong with the aircraft, like water in the fuel, ect....