If someone or something bears down on you, they move quickly towards you in a threatening way. A group of half a dozen men entered the pub and bore down on the bar = advance
move forward in a threatening manner (e.g., "The cruise ship bore down on our canoe"); use utmost effort and concentration (e.g., "If you really bear down, you will success"); contract the muscles of the abdomen during childbirth in order to facilitate the delivery
To bear down on something means to push or press downwards with steady pressure. The roof support structure had collapsed and the entire weight was bearing down on the ceiling