Binding constraints at second-best optimum
I think its not a complex issue: 

a) You need to keep the high type agent from pretending to be the ow type agent, so you give him an extra compensation from showing his high type. But this nice deal pays him more than necessary to keep him in the game so his IR is more than satisfied, i.e. not binding. 

b) Similarly, you will make it painful to show that you are a low type, so that the high type doesn't go there, but you can;t make it so painful that the low type doesn't want to play so you pay him the minimum amount that he will take, i.e. his IR is binding, but  he would never choose to show himself as the high type anyway because with his low type, the incentives don't work in his favor, so the IC is not binding.