Such a transfer sounds very unusual and I have no answer. My best guess is that your veteran had some specialty that the 99th needed.
The "Bulge" began on Dec. 16. The ;st Div. was in a rest area behind the lines.When things got rough,. They were cslled up in support of the 99th and 2nd Inf. Divs.
At the ed of the Bulge, we met several African-American GIs who had recently joined the 1st Div. They hsd been Sgts. in the Quartermaster Dept., mnostly truck drivers, and they had voluntarily given up their stripes to fight as privates on the front lines. This was one of the first integration in the arned services. This has nothing to do with your situation but I thought you might find it interesting.
I will e-mail you more on my experiences in the 99th.