Certified GKH Gardening Expert
A number of things could be causing this. For instance, it could be due to extremely rapid growth, usually in periods of abundant moisture after drier growing conditions. Likewise, it could be attributed to squash rot from a calcium deficiency. Pests can also be an issue, such as squash borers which are difficult to see as they usually burrow within the stems.
Here is an article or two that you may find helpful: https://www.gardeningknowhow.com/edible/vegetables/squash/squash-blossom-end-rot-causes-and-treatment.htm