I believe this is a known bug called the "scrolling bug" and it's been captured in a screen recording and hopefully officially submitted (saw it in a Lemmy post earlier). I don't know that the cause has been determined, however.
I found that I only encountered it when viewing 'All > Hot' or 'All > New', I believe. I thought it was someone spamming the server with the same thing to different subs, but I see now that it's a bug.
So for now I stick with 'Subscribed > Hot' or 'Subscribed > New', and I subscribed to a crapload of subs.
I noticed this as well and thought it was meant to be a feature at first. Real time updates and just going with it. Glad to see it's apparently a known bug!
I would also appreciate it if things would generally just stay still and stop updating while I'm trying to look at them, especially the posts timeline.