Why would web page process continue to show up in Activity Monitor after quitting the web page?

Running Big Sur on a 2017 Intel iMac, Activity Monitor shows a process for each web page tab open in Safari. Normally these processes end when I close that tab. examples: https://support.apple.com https://apple.stackexchange.com When I visit most sites, the process shows up, and the process ends when I close the tab for that web page or just type in another web address. But for one site, the web page hung in a frozen non-responsive state, requiring me to force-quit Safari. Strangely, the process kept running in Activity Monitor and consuming a large percentage of my CPU even after had closed Safari! This was a well known bank, https://53.com . I killed the process in Activity Monitor and tried to replicate the problem but now the page behaves properly. When I close the tab, or just choose another web site bookmark, the 'https://53.com ' process shuts down in less than 10 seconds. I am not running any extra advertisement killers or even the available Safari Content Blocker preferences. I do have Pop-up windows set for Block and Notify by default. Just wanted to ask if anyone can shed light on this behavior.