The most straight forward explanation, to me, would be the change to tons of heroes paired with the switch-around of the number of bans per round in -cm. Teams are faced with difficulties in drafting since many of the old popular first-picks have fallen out of favour, and both teams are able to guard themselves to a lesser extent. The Radiant (who defaults to firstpick unless settings of lobby specifies other) statistically wins more, even when teams are randomed a side. Seeing no map changes, this seems to imply that the first pick is that much more potent now as compared to 6.74.
I personally expect people to learn and adjust, and with them - the win rates. Give it a month or two and the win rates will with most probability be back to 50/50 (or close thereof). I'm just interested in why they're currently so uneven since this has never happened before.
Apologies for the whole broken record thing.