The STM32 unwantedly locks or it goes on the bootloader mode

[VanitarNordic]

I have an STM32F103C8 chip.

as we know and it is written in the datasheet, if we want to load from flash, we have to pull down the BOOT0 pin with a 10K resistor.

But in my case, when I pull it down, after a few flash erasing/programming, the MCU suddenly locks. Then I have to pull it up, go to the bootloader (using USART1) and erase the flash to unlock it. It was happening frequently till I left the BOOT0 pin float and after that nothing happened and everything is normal.

Why this happens?, weird