To give an example:
You are looking at batch X of replies (say, 20 in 1 batch). You scroll to the bottom, where it loads the next batch (batch Y). Instead of allowing you to keep scrolling to read the replies in batch Y, it jumps to the bottom of batch Y where you can see it’s loading batch Z. And this continues. You can scroll up to read what happens in that batch you are in (and previous batches), but it will jump to the next batch anyway.