Probably due to its too close to the stem, as you may know the hormon and nutrient concentration is too high there. Another reason would be some cucurbita families have dioecious flowers in the same plant if you do not remove them the test of the other fruits would get bitterness
As far as I know there is no approved reason for the bitterness of cucumbers, even in our culture there are many believes but non of them has been approved academically. But from what I have read it is saying like that and that is all I know about this. Sorry