unexpected reset after changing the GPIO mode on Fly.

Dear experts,

my STM32F042G6 is unexpectedly reset after I switched on fly CAN_TX GPIO to output GPIO pin.  

for this I did this steps.

1).HAL_CAN_STOP() and then 

 2). GPIOA->MODER &= ~((U32) "pinX");

   GPIOA->MODER|= ~(U32) "pinX");

I have also made the following steps.

1) . HAL_CAN_DeInit(hcan)

2) . HAL_GPIO_Init(x_GPIO_Port, &GPIO_InitStruct);

the result is the same. 

I have looked at the reset register (RCC->CSR). I can't find any difference to normal power on . 

after Reading (RCC->CSR) i become this Value : 0x0c80 0000

that mean : 

1) Bit 27 PORRSTF: POR/PDR reset flag

2) Bit 26 PINRSTF: PIN reset flag

3) Bit 23 V18PWRRSTF: Reset flag of the 1.8 V domain.

also set a little breakpoint before changing GPIO mode and go through the lines Step over then there will be no reset. 

but if I am in debug mode without breakpoint but through GPIO change debugging tool breaks.  

maybe somebody has an idea why this reset happens or how I can find out what he is causing the reset. 

thank you