How is the tab chosen when opening a link from an external location? I have Safari open with 111 tabs open.
I clicked on a URL that a friend sent me on Skype.
Safari opened the link in a random tab that wasn't selected, and was neither the first or the last one on the left or right.
The URL that I clicked replaced this tab's content, it was not a new tab.

How was this random tab selected out of the 111 tabs, to be the recipient of the URL I clicked?
It's usually not a random tab but the current active tab. But with 111 tabs open, I'm surprised the app's memory is not hosed so much that it's making mistakes now. That count seems a bit high. Try to reduce the tab count--after restarting Safari--and check the behavior then.