Most of the clips I've seen only show the incident and not a wide angle shot in the seconds leading up to it. From what I can see Cavendish looked twice - 1 good look then a quick glance. I think the Korean just moved quickly out of his blind spot into his space. It's happened to me on a few occasions in the past. You always take a risk when you choose to move through on the inside of another rider at speed.
Agree with this - and isn't Cav looking at Viviani anyway? Going up the inside of a rider half way up the banking without being ready for a move is short sighted - you know they're coming left at some point