The angle you see it from on the tele is slightly misleading. The Korean had overlapped Cav and moved up towards him whilst looking the other way, just as Cav who was almost a bike length clear moved down. Viviani claimed it was not Cav's fault but a racing incident.
If Cav was to be penalised, then the move where Viviani put Boudet into the infield (the difference being that Boudet was looking where he was going and was able to take avoiding action) would also be subject to sanction.