Most likely for the same reason tomatoes get blossom end rot - a calcium deficiency. Calcium is necessary for plant cell structure. If the plant can't uptake sufficient calcium, the plant cells will collapse as the fruit expands and matures. There could be several reasons for this including, uneven watering, unavailable calcium, magnesium or other vital nutrients.

To prevent, use a fertile & moist soil and mulch.

Lack of pollination can also be the cause, It is surprising how long the young fruit can grow when not pollinated, but sooner or later, it catches up to them.