I don't know if this is relevant to this situation, but I noticed BS Player (2.51 in my case) returns to the foreground if you click another window before waiting a few seconds (4 or 5). Turning off "Auto hide main window" in General/General settings stops it from happening. It seems like a bug and may have been submitted already (haven't checked).
EDIT:
You actually have to do something to the player such as moving the player window for the bug to occur.
Apparently it's a very old bug:
auto hide main window problem