Few Questions about Diet, Back Training and Squats

  I never deadlift due to an extreme concern for my lower back. However, I just started to re-incorporate rack deadlifts into my back workout to help build my upper back. Yeterday, I pulled 225x8 315x8 and lastly 405x6. I usually front squat with a last working set of 225x5.    Today, I could barely do two squats. 
 Also, I just started doing a low carb, 3xweek HIIT and 30 mins elliptical after each lifting session. I know that the lack of carbs and cals can make me weaker, but it's only been 3 days and  I really felt very weak today.
 Lastly, I've never been sore in my upper back EVER from rackpulls. I just have this sh*tty soreness in my lowerback for the next 4 days and that's all. WTF?! 
                                                       Thanks guys

You’re doing rackpulls wrong, eat more food, and understand that your body fluctuates and there will be some days where you just aren’t 100% because of an endless list of factors.

I agree about the doing rack pulls wrong.

Where are you pulling from OP… how close to the kneecap? Above or below?

If i am incorporating them purely to increase my pull - i progress the depth of the pull, starting above the knee but if i use them for simply training my traps - as they do VERY well for myself, then above the knee is fine.

I also like (really when using them for muscle and not raw lifting strength) to retract the shoulders at the top of the move - not as a separate move as many do, but all one ‘fluid’ motion.

It is a static contration on the mid/low traps (etc) as you pull so you are just pulling them back further towards the top as the load is shifted off that area slightly.

Your front squat lift is really good though IME.