To the post before this on on the cutting of the panel.....first the carbon fiber above post they look fantastic on your car!
I am not sure if you are aware of what is right under the plastic panel you have cut.....there is some sound deading insulation, not water coated and in fact your (metal) rocker panels interior. You will need to get back in there if you have not done so and use some form of waterproof silicone to coat that open exposed area.
Otherwsie if water can seep in there and it will....your going to have a rust problem. If you have seen what is under that panel with it not being there your eyes will roll in the back of your head.....you might as well through in a wet towel. The insulation will wick the moisture and then let the problems begin.
The reason they crack is that the portion of the rear and front rocker where the two screws go are not at 90 deg. they are more like 110 deg so when they tuck the rocker by those screws the over extend, are bent inwards causing the rockers to stretch thus splitting down the exterior joint.
The simple solution is to back out these self tapping metal screws so there is no tension, no they will not fall or back out, and have them set so they just barely meet the plastic and the problem is solved. Also be aware that on the rear of the rocker covers are three plastic clips that set the rocker cover into the rocker panle and two adhesive pads that secure them too. They are not going to fall off, it's good to make sure you covers are also snapped properly as they have arrived with them not being snapped in place sometimes
Just back out the screws and the problem is resolved.