Why does Cubase automatically set event start locations?

Hi,

The Event Start is obviously placed at the start of the Event, as shown in your screenshot.
You have even added a red box to highlight it.

I believe you are talking about the Snap Point, this sets where the Event will snap on the Grid.
Indeed there is an issue where the Snap Point is placed to the end of the Event under certain circumstances. If that’s the case, then simply open the Sample Editor, find the S handle, and grab it where you want it to be.