Why did the Knight Cole attack? We don't even seen him losing his patience or what was the ultimate trigger point to what was boiling inside him. For an episode of television that just felt weird.
Between Rhaenerya rejecting his marriage proposal, being forced to watch her and Laenor being presented as a future couple, Criston Cole's ego had been somewhat damaged. But I believe Ser Joffrey's comments about their situation finally triggered his violent behavior. Frankly, I think Knight Cole had been very stupid, illusional and immature.