It's really quite f'n simple.
We did not have Barisic, and as Gio anticipated Dundee utd playing 1 up top (fair assumption), he favoured having 2 full backs in their natural position and a "replacement/covering" CB next to Goldson in order to aid that (Proving Simpson is no where close to the picture frame but that's another story).
When he saw they were playing with 2, he clearly thought that was more of a threat centrally than he was comfortable with, and conceded the left hand side a little to ensure the centre was more solid.
All completely fair and part of a managers remit. Nothing baffling about it and proves our manager is very proactive and most importantly, reactive, when he has to be.
Some really silly comments on this.