It seems to happen at random Kate. That looks like it might be a mature rosette. I have a few every year that do the same things. Looks like you have many nice replacement plants to keep the line going.
You can try cutting the center bloom stalk off down low and removing the blooming offsets. Some times it will trigger new growth the the stolons. It's worth a try if you don't have replacements.
One of my Red Rubin did the same thing last summer. This one had about 1/2 normal offsets and the rest went to bloom. I had some that all went to bloom like your semp is doing.
