Grill Transferring is a surprise and I wish him the best.
Before we freak out though I am not sure Caleb was even a year away from being a major contributor at ISU. At that point he'd be recruited over (which many demanding JuCo/transfers beeing added would have happened). This year Grill was forced into playing due to the injuries and other freshman transferring out not due to him being ready for B12 play. He may feel more comfortable playing at the non-P6 level that he originally committed to. There are many reasons Grill may feel ISU isn't a fit for him and he has the right to try some place better for him.
I am not giving Prohm a pass though, losing 3 out of 4 recruits in a Freshman class is a huge problem. From identifying, prioritizing and development of the players it is his job to have them ready to fill set role and it didn't happen. I can't believe Jamie will take this attrition lightly and will be using this upcoming season to decide on staying the course or going a new direction. This off season will be the most important of the Prohm tenure, if he doesn't replace this class it will be ending shortly.