Safari on Yosemite stops loading tabs if a new one is created
I've had this happen every time with OS X 10.10.2 and below. It has somewhat lessened with the 10.10.3 update. But it does still happen. The only way I've found around it is to make sure some of the site loads on the tab before creating a new tab (or even switching to an existing tab).

Sorry, I was hoping to just post this as a comment, but not enough reputation yet.