Your knees aren't sliding forward much in general, though on the last rep they do a bit because you didn't get your knees forward into place soon enough. So, that might be what you need to keep an eye out for. Side note, try and take a more controlled breath each time so that your entire torso doesn't shoot up and down again. Also make sure you are bearing down hard on your abs before each rep - your back seems to be a bit arched here.