Your rear hatch is not fully latched. I struggle with this on my CRV and Pilot. Make sure nothing is in the way when closing. If I go to pull up on the door to check that it is closed, I usually touch the latch pad and open it just enough to get the light to turn on. Pressing on the door to fully close usually does not work either. I have to open and shut again making sure I do not engage the open pad. From looking at the wiring diagrams, the door open alert is based on the latch circuitry going back to the computer module. No real sensor or switch outside of that.