I can explain what I heard in more detail but I can’t document it, because as I said, I don’t know the source. There is a machlokes Rishonim what constitutes “Chupa” to effectuate a nisuin. The Rambam holds that the Yichud is what creates Nisuin. The Ran and others hold that the canopy that we use is the chupa that creates nisuin. Here is one version that I have heard.
The Rambam holds that “birchas nisuin” has to be made before the nisuin (i.e. before yichud). Ishus 10:3.
The Ran holds that “birchas nisuin” should be made under the chuppa. Pesachim 4a (in dafei haRif).
So what I heard is that the minhag is to be mekayem both shittos. The brochos are made under the chuppa as the Ran holds, but because according to the Rambam it is Yichud that is chuppa the brochos are properly being made before nisuin.
However, there is a problem of hefsek between the brochos and the Yichud according to the Rambam. So in order to demonstrate that the brochos under the chuppa are for the yichud too, they hold hands from under the chuppa until they get into the yichud room.