Hand position:
You might have your hands a little too on top of the bar. I have the bar snug on top of my thenar eminence (the meaty part of the drumstick if your thumb is a chicken leg). It looks like yours might be resting on top of the bar. I don't know if that'd make a difference for your pain, but that's just the way I position the bar. They way you have it (I've done it like that before) can lead to wrist flexion, even if you aren't flexing your wrists for now.
Related (time stamped):
YouTube
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Thoracic tightness:
You're staying tight
through the rep, and it looks like your elbows are in a good, neutral height during the descent and ascent. Nice work. However, it looks like you're relaxing your thoracic at the top (when you finish the rep). You'll notice that this (relatively) causes your elbows to jack up a little every time at the top. This may wear your tightness down a little in the later reps, so keep it tight at the top, through the whole set. In other words, if you let it relax between reps, chances are that you won't get it the same tightness in the 5th rep as you would in the 1st. This fatigue can make you jack up your elbows through the rep (I'm guilty of this).
Try to make your position on the left possess the tightness on the right:
difference.jpg
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Also, unrelated, your head is coming up when you're driving up, especially on the last rep of this video. Keep it down (tennis ball tucked under chin) to help stay in your ass.
That's all I got. Keep working and doing your stretches. Hopefully your body will adjust after a while if nothing else.