I think they're dying, this happened to one of mine recently, and at first I thought it was turning yellow because of autumn or something, but it went mushy too. I pulled it out, you've got lots of healthy rosettes there so they'll soon spread out into the spaces left.

Did those two flower? They often die off after, but as Louise says there are usually lots of little ones around to take their place.

Hi
No these two particular plants didn't flower they just kicked out loads of rosettes. Thanks for the replies.

Let them finish dying (if, indeed, they are) and allow the rosett to go brown and dry. This will allow you to pull it out, with your fingers or a pair of tweezers, more easily and without doing more damage. The smaller rosetts will soon grow to fill the gap and you could put a bit of grit into the holes meantime.

Do check that there are no Vine weevil grubs under the dying ones.