Pin value movement for the pin 38 during booting

I am using BPI-M1+ with 40 pins.

Pin 38(GPIO20, PI20) moves to high and stay for about 100ms and goes down to low during the very early of the booting. This phenomenon happens very randomly, not always, so I cannot even guess what the problem would be. Sometimes it happens once in 15 times try, and sometimes once in 30 times try. Below is the picture of signal from two pins(violet: 3.3V Out, blue: pin38)

The power goes up in 3 steps. It seems that the pin 38 goes up to the second step with the power output pin and goes down.

It happens when I connect the power to the banana pi without a memory card. I am using the pin 38 as a alarm signal, so I need the pin to stay low without any alarm occured. I do not know what is causing the problem. If anyone has seen this kind of problem, please help me! Thanks!

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