I've had this problem with a yellow and also with a dark blue, both VGC. I finally opened up one of them to check, and it appeared as if the pigment had just collagulated, leaving the medium around it. When squeezing the bottle, you just remove more of the medium and the pigment stays right where it is.
A found that a telltale sign is not hearing any sloshing of liquid when shaking the bottle. Which is why I always shake a bottle in the store before buying it. Also, if the clot is large enough, you can actually feel it shaking inside of the bottle, and I'd advise to leave it and pick another one instead...
I do suppose though, that opening the bottle (by removing the top nozzle) and stirring/poking the clotted pigment till it breaks up and mixes with the medium again.
It has nothing to do with drying out (there's plenty of medium present), but maybe with aging of the product?