SDIO signal affecting ESP32 boot

[tchen61]

i am trying to test SDIO slave driver. However, i found that if the signals are connected to a SD card slot (CLK, CS, CMD, DATx), ESP32 (i am using ESP32-pico) seems to boot in a strange way (ie not launching my main program). however, once i disconnect those signals, then it boot up normally.
It seems that boot loader is using these signals for boot, how can i disable it ? (i want to boot off flash, not using SDIO signal)

thanks
tom