Why Are My Butternut Squash Splitting

Question

They are not ready to harvest. They still have some green on them.

Answer 1

Uneven watering or temperatures are normally the cause of fruit split in plants. Stem split is also common and usually due to extreme temperatures changes and uneven watering as well, but it never hurts to check for signs of pests, especially since they can penetrate the plant through these splits.

If you feel you need to go ahead and pick them, they will ripen off the vine. This article will help you:
https://www.gardeningknowhow.com/edible/vegetables/squash/ripening-green-unripe-squash.htm