arrow buttons

New Mac user, so I'm still getting the hang of it. Sometimes when I'm on a website and using the down arrow button, it moves the screen down incrementally (scrolls down) so I can read an article, etc. but sometimes when I'm using the arrow down button it jumps all the way to the bottom of the page. When it does this, I can't get it to scroll incrementally again. Why does this happen and how do it fix this issue? I don't always want to use the trackpad to scroll down a page.

Thank you.