Kellner Too Petrified to Call?

There was a raise from the gent on the button and in the small blind Jan Petras called, before Pavel Kellner in the big blind reraised to 5,000 or so. The original raiser folded but after a little spell in the tank Petras called, and they went heads up to the flop.

Flop: :::Kd :::2c :::Tc

Petras checked, and Kellner continuation bet 5,125. Again, Petras tanked – and again, he called.

The turn was the :::4d and this time Petras bet out 18,000. Kellner clearly was not expecting this and looked rather unhappy. He folded, leaving himself  just 22,000 or so. Petras meanwhile is doing just fine on 60,000.