What am I doing wrong adding this new switch to the stack?

Okay I've stacked a ton of Cisco switches, though I've technically never just thrown a new switch into an existing stack.

I've got a site that had an extra switch they wanted to add into their existing stack of 3750X switches. It already had a previous config, but from my knowledge and everything I thought I interpreted reading on that, you should be able to throw that on there and the current master should just default it, as long as I didn't lose power to the stack or something.

When you add a provisioned switch to the switch stack, the stack applies either the provisioned configuration or the default configuration to it.

However, when I did this, it decided to take over master, wipe the config and give the stack its own previous configuration. I figured, yeah I'm being sort of lazy I should have done it right. So I got things back to normal, had the customer remove the switch, wipe it, set priority.

Connected it again, bam takes the stack down again and wipes them.

How is it taking over this stack every time?