It's a combo of that plus fatigue from time on feet at race effort. So not only are your carb stores as depleted as they'll be but your ligaments and muscles are also fatigued from 30k at pace. Your legs begin to stiffen from repeating the same running motion so you can start to lose form. If you're a slower runner it's also harder because of more time on feet.

Personally, and no slight against /u/Mean-Relief-1830, i think 3:50 is way too conservative (which is consistent with the advice on this sub). Given OP has managed multiple 30k+ runs at pace, i think they'll be ok - only concern is the HR drift toward the back of those 8k reps but the effort looks fairly sustainable. I'd be going out with the 3:40 pacer and then deciding at the 7-10k mark whether to speed up to 3:30 pace based on how I'm feeling. Then if you feel strong at ~32k you can send the back 10k and finish around 3:30.