Why are my potatoes cracked?

I had baked potatoes for dinner today and was disappointed to find they had big cracks inside. They looked perfect on the outside (and tasted fine!). I suspect that this will mean they won't store very well. What is likely to have caused this?
We had a fairly cool summer this year, maximum temperature about 22 celsius, (72 Fahrenheit) but usually quite a bit less, with no prolonged dry spells. Reasonably normal for my area. They were planted in a new growing area which turned out to be rather acidic with poor soil nutrients, but the potatoes got old sheep manure top dressing; some when planting, some a little later.
I've eaten the evidence, but will try and take a picture of some other tubers soon.