The photo above shows a fairly advanced case of a cabbage splitting. You may notice it at an earlier stage, when it looks as if one or two of the outer leaves are being torn in half. Unfortunately, once it has started to split, there’s no way of stopping it (that we know of) and if left to its own devices it will soon look like this one.
Splitting happens most commonly once the cabbage has more or less reached full maturity, and usually begins after a heavy rainfall following a dry spell. The cabbage essentially draws up too much moisture and grows too fast, causing the splitting. You may find the same thing occurs if you over-feed your cabbages late in the season, with a liquid feed, for example. It may also be that this year, in particular, the warm temperatures in the past week or so following a relatively cool summer may have exacerbated the situation.
As is often the case in the veg patch, the best tactic is to try and prevent this from happening in the first place, and that ultimately comes down to keeping the soil consistently moist rather than letting it dry out and then get saturated. This dry-then-wet-then-dry in the soil is also the main reason you would see blossom end rot in tomatoes, or split carrots. The best advice is to water well and regularly, and to apply a mulch to try and keep the moisture locked in the soil.