I have no idea so I'm just speculating here, but it seems like an easy open goal to help such a powerful rider for a bit in his moment of need. I mean hell, if the yellow jersey Tour de France runner-up 2x Vuelta winner is riding by all cut up, bruised & you're just sitting there twiddling your thumbs, coasting home with zero objective, why not help a little?
With regards to Nacer Bouhanni (who has been mentioned a few times here), A/I don't believe he even did deliberately help Roglic, he just thought he was the "ticket" to get back into the peloton & perhaps go for the sprint win & B/if he did deliberately help him, an easy logical step would be to say his own bad time at Cofidis (who were drilling it up front for Laporte) "could" have had something to do with it.
But it's all just idle speculation & in any case, Bouhanni blew up in his wheel & one of the other guys in front of Roglic (I can't remember his name) almost led him over a cliff into a ravine. So he did the vast bulk of the chase all by himself, basically.