If the iPad has enough RAM, it tends to keep the last state of an app in RAM; so you can switch back to it quickly if needed. I will usually do this for the last 3 to a dozen apps; depending on model, available RAM, and the size of the apps involved.
Normally, this doesn't mean those apps are running. They are just sitting there, frozen, waiting. If another app is opened that needs the RAM the oldest frozen app(s) will be purged to make room.
If you are going to a new site by tapping on a link in another app, or by using a home screen shortcut, it wouldn't be unusual for Safari (or another browser) to temporary show you the last site you were on before the new site has started to load.
If this is something happening when you are already in Safari and choose another link (from a bookmark or favorites), I have no idea what's happening. Not unless your link is some kind of redirect from the site first shown.