I believe he left KSW to fight for One, but I don't think it went the way he expected, and I'm not sure if it was a smart move financially. From what I know, KSW would offer really decent money to their top stars, and Soldic was one of them. Or maybe he was just hungry for a new challenge.
He must've been offered decent money for the Adamek fight and, most of all, it's pretty much risk-free. It's a boxing bout against a former boxing champion (well past his prime, but still) so it wouldn't be any shame if he loses it.
Soldic has some boxing background and I think he had some official fights before moving to mma.