Hey Seth,
Is this happening on the stock CRM_EditCustomer screen or on your customized version? If it's happening on your customized screen then edit the screen, click the Advanced button, and look at the actions under the OnScreenLoad event. You'll want to look for the Show Screen in Pop Up action, normally it would be contained within a Conditional Statement, so I'm guessing in your case it's not. Below is a screenshot of what the actions normally look like.
If this is actually happening on your stock screen then please enable the MethodSupport user and email me your Company Account name. Go to Customize > Users > scroll down to the bottom of the screen and ensure there is a checkmark in the Active box for the MethodSupport user.
Additionally, in order to have the Transactions tab section show first instead of the Contacts tab section, you'll need to use the Go to Section action and just select the Transactions section.
- Ashur