A common mistake seen in many people who do bridging is a rib flare. This is when your ribs lift up at the end of the bridge movement. When this happens your abdominal muscles are put on a stretch and the load goes straight into your lower back and not into your glutes as it should.
Often this happens when you try to get more movement at the top of the bridge, particularly if your hip flexors are tight, the movement lacking at the hips you would try to gain via the upper back causing rib flare. This definitely needs to be corrected if you do bridging regularly otherwise you'll be straining your lower back and not gaining any strength in the glutes and abs at all.