Safari is pretty trigger-happy now when it comes to websites that do weird things. When you see the "using significant energy/memory" message, it's Apple's way of telling you the website has some weird scripted behavior that causes it to require more processing power than usual.
But what "usual" means is unclear. I'm guessing they detect to see if a website continues to use the CPU after it has finished loading, and if it does, then "using significant energy" is shown. The same thing may apply to memory: if memory usage keeps climbing up after the site has finished loading, then "using significant memory" is shown.
This is on the website developers to fix, and not on Apple. You can try a different browser that won't give you the message if it annoys you, or... just dismiss it and don't worry too much.