Pin mux during bootloader and reset press

Hi, I’ve noticed that when you press the reset button on the BBB pin P9_17 and P9_18 are raised up and then they go down when you release the reset button. Same for P9_15 and P9_12 but those two stay turned on until kernel launch the device tree. For P9_12 and P9_15 i’ve figured out that this is due to uboot pinmuxing (P9_12 is set to mode7 and P9_15 mode0 gpmc_a0) but i’m not sure why they are toggle during bootloader or reset press.
Does anyone know more about this and if I can change the pinmux for those pins in the mux.c without any risk for the board ?
And I haven’t found anything about I2C1 pin toggle when you press the reset button.