Problems with cabages

I have only recently started an allotment and this year had been obout clearing the land. However I noticed some cabbage plants for sale on a market stall which were labeled spring harvesting so I bought some and planted them. Six weeks on and they have grown to be huge and appear to be splitting open.  I thought at first they were being eaten by rats but on closer examination it looks as though they are just burstin open.
Has anyone come had this problem befor and is it common?

They do seem to be a very fast-growing variety if they have huge heads just 6 weeks after planting out. Which variety are they?

As mumofstig said, they sound to have been ready for eating all at once. However, most varieties of cabbages that I have grown will happily stay in the ground for quite some time, when fully grown, without splitting. Having stated that, I have experienced splitting with a pointed variety called Hispi this year and, to seemingly make it worse, loads of earwigs took up residence within them. Although the cabbages were fine to eat, seeing the earwigs drop out of them in the sink was a tad off-putting.