ArduinoMega DigitalPin51 DigitalRead become HIGH

DigitalPin50 is MISO(Master In Slave Out)
DigitalPin51 is MOSI(Master Out Slave In)
DigitalPin52 is SCK(Serial Clock)
DigitalPin53 is SS(Slave Select)

Maybe MOSI is  automatically overrides to OUTPUT.

ArduinoMega's PinMapping