It’s because you have several “windows” or “scenes” open with Safari. Swipe up the one you’re not using to close it, and it won’t show the window-drawer when you switch to the app; Though you can always click the already open app’s icon in the dock to make it show again
If you invoke the window-drawer, either by tapping the app icon when it’s already open or by holding on it for a moment and then hitting the option “Show all windows”, there’s an extra one to the right of open windows with a little plus on it to open a new one. You can also drag another window next to the current one, split view style, by just dragging up the icon
You’re welcome - Multiple windows have existed for some time, but this way of presenting them with the little drawer at the bottom is new to iPadOS 15 too