I see the slight light change.
I suspect any type of light change is going to set off the motion detection. (clouds, etc).
I have an outdoor cam - and it's been snowing there, and I have about 6 totals blocks in the image - and I am getting massive amounts of motion detection everytime a flake goes through the area.
The camera log file shows 600 since 5 hours ago.
629. 2017-01-28 18:12:55 MOTION STOPPED
630. 2017-01-28 18:12:54 IP CAMERA Received MOTION Trigger
I don't remember if that camera has PIR detection, but that might work better in your situation.