You must not be a skater. Mariah was doing her program run-through to music, placing all the choreographic elements where they occur on the ice just as in actual competition. Her program has places where the choreography goes close to the boards, and her leg is extended out to the side, and in some cases moving in and out (leg not stationary but choreographed movement). Eunsoo happened to be physically in one of those locations at the wrong moment, and got her leg grazed. There is no way this could be deliberate, as Mariah would risk not only throwing off her entire program, but also injury to herself. In official practices, once you are doing a run-through, you don't stop unless you are injured, because you don't get another chance at redoing or starting your music over. It's possible (probable) that Mariah wouldn't have even noticed a slight graze as she was flying by, focusing on her music and elements.
You need to understand that by universal custom in the skating world, the skater whose music is playing and doing a run-through has absolute, 100% right of way and everyone else on the ice must yield and stay out of the way. That means in the middle, in the corners, by the boards, or anywhere on the ice where the program skater may skate. It was Eunsoo's responsibility to stay aware of this and keep clear. For whatever reason she didn't--and I'm inclined to think charitably that Eunsoo just momentarily wasn't paying attention and got mentally focused elsewhere--it happens frequently with skaters especially at high-stakes competitions.