The aquacraft esc bec component lacks "mirror image protection" so the power bleeds through its bec from the receiver supply.
DM and myself opted for the higher spec, higher priced LM2940 which will not give the same symptom. More importantly, it will not be destroyed if you connect a receiver pack without disabling the bec, either by pulling a jumper or removing the red on the servo plug.
Refer to the appended image. Without the leftmost 1N4002 the 7805 regulator can feed 4v to the 12v side if you remove 12v then connect a receiver battery to the 5v, as you are doing. As an aside, the second 1N4002 provides an alternative path to protect the 7805, during discharge, if an external 2,000 microfarad cap buffers the 5volt line.
I have not had time to ask the author for permission to use his image. I'll delete and later upload my own if the objects.
Browse his site if you are curious.
http://www.williamson-labs.com/480_dec.htm