Disabling the antivirus or turning on silent mode may help for sometime but if you restart the system it may delete it again. There are some dedicated scans which check for PUPs(Potentially Unwanted Programs) & delete them no matter if your antivirus is turned OFF. I'm using Avast antivirus which caused me a lot of pain to fix this. There are multiple scans i have to configure to exclude the folder & bfgminer.exe from scans.

The best way to resolve this is to add exclusion rules. I would suggest check the documentation of bit defender in configuring the exclusion rules. Hope this helps