If this happens on a third party website, then that site might be programmed that way. In my experience, especially review sites seem to want visitors to watch a video that is pushed, often not directly related to the subject of the page. They may get paid for showing the video, per viewer and minimal viewing time. Depending on the site, there may or may not be ways to block that (on iPad).
If it happens in a different app than the browser, then please specify.
If it is overlayed outside of any app, then it uses Apple’s PiP.
The setting for that is in: Settings﹥Multitasking & Gestures﹥Start PiP Automatically﹥On|Off
Multitask with Picture in Picture on iPad - Apple Support