Hello Liam!
This is simply because when you went to spawn in, you and another person must have been going for the same frequency at the same time and they got it first, causing you to observe their frequency, rather than control it. I see you were going for KLAX, and it can get pretty busy there sometimes, so you can expect this type of situation to occur. If you’re looking to possibly improve your ATC skill and ability, feel free to open a smaller airport, and maybe even a tracking thread, and us IFC members can come fly for you. For guidance on how to make a tracking thread, check out this thread :