Purple sprouting broccoli

Unless I am mistaken, isn’t this usually a spring crop? I usually look forward to it in April. I can’t remember the variety but I thought they were all spring flowering. However, my 4 plants are currently sprouting delicious broccoli. Whilst I’m not complaining about enjoying some now, I presume this means that the plants (about 3 feet high) will die after flowering and therefore I’ll have none in the spring. Is this correct? Why would they be flowering now? Is it just the weather or could it be something I’ve done/not done?

Last year mine (I think Rodolph) started producing florets around this time but stopped fairly quickly once the weather 'turned' and went on to behave normally.