This is what you call a “Wolf tone.” It happens more often on cellos but can also happen on violins and violas. I’m not sure the mechanics of it, but is an extremely annoying thing to deal with.
Most repair shops can take care of this issue by adjusting the sound post of the violin. This is definitely not something you would want to mess with. The other option is to get a tool that helps prevent it like this wolf-be-gone. They don’t always work but you can always return it if it doesn’t do the trick.