If radishes start to get these "holes" (I only know the German term, sorry, edits welcome), it means woody cells have started to form and the plant is preparing to flower (= starting to bolt). A common cause can simply be that you
- waited a tiny bit too long to harvest and honestly, they seem quite large for the oblong type, grown outdoors, IMHO. Typically radishes mature in 21 to 28 days, especially during summer.
- Irregular watering, high temperatures and sometimes lack of nutritients can speed up this process, but that seems not to be the case here - taking the size into account. (But, on the other hand, don't fertilize too much or you might end up with a lot of leaves and small radishes - at least radish leaves are edible, too.)
- Some breeds have a higher tendency to get these holes [whatever the correct term is] than others, so if you are botherred by this, switch to another breed next time.
If your radishes taste fine, there is nothing to worry about, but you should harvest and use your radishes soon, before they get really "woody" streaks that are no fun to eat.