Processing 4K videos can sometimes cause these issues, especially if you add several of them to a single project. This happens because they require a lot of resources.
You can try changing the resolution with special programs that help keep the quality. This will reduce both the final project size and the resources needed from your device and the app.
You can also remove the video from the project and upload it again. When uploading, the app automatically compresses the video to make the file smaller.