Heavy squats and heavy deads on the same day is doable, and is part of the Rippetoe workout, of course, but deadlifts are hard on the body, and after three sets of heavy squats I could see your form fading a bit. A few other things -

Are you taking any kind of pre-workout carbs with your shake? I don't know why it would only affect one rep but it could be that your workout ran a bit long and you started to crash. Just a thought.

Also, what's your form look like on deadlifts in general? I've had it happen that I did heavy squats with good form, went to do deads, and just couldn't maintain my usual form. So it could just be fatigue.

Good luck.