MySQL All Passwords Are Changing

[TonyGuy]

I have a user login panel on my website connected to a MySQL server. However, in the last 2 weeks, over 240,000 users' passwords have all started changing simultaneously. This has happened three times already, and all passwords were in md5 format as follows: 865a6381d7215254574c0b745d790433, 456ea569a0a5c4b55d8be165df3e09eb, and dece3413d608ea5e3452f5a3dca6f08f. I back up daily, but the issue persists. Is there anyone who can help?