When cutting a section of a celery plant stalk it appeared hollow inside, i.e. the cortex was hollow and encircled by the cambium, the vascular bundles, some cortex and the epidermis layer, thus the structure appeared similar to the celery stem's one and to the one of many other vascular plants, but the cortex. I have struggled to find answers on this topic: the cortex has a pivotal structural role, since it becomes turgid and keeps the plant straight, not to mention its water storing properties. Then why would it be so thin in such an important part of the plant, (stalks develop into leaves)?
Celery is a wetland plant, and likes to have a constant supply of water. Other vegetables can "bounce back" after experiencing a short drought, but celery usually has more difficulties. If the plant is young during reduced water, it typically will become dwarfed, and fail to grow to full size. If the plant is older, it will develop defects, and suffer long term damage such as what you have described.
Defects are also possible if the celery does not have sufficient micro nutrients, or if the weather is very warm during growth.
In short, this is caused by non-ideal growing conditions of the celery. The celery was stressed to a point where it sustained damage that it could not repair. Celery is one of the more difficult crops to grow because of how finicky it is to drought.
-Personal experience trying to grow celery