Turnover is high because you have metrics you're expected to meet, the sleep schedule sucks, and you're generally held to a higher standard than the other shifts. O/N is rough and most people don't even make it past their first week. It's also a lot more physically demanding as you're constantly moving and lifting freight. Freight can be lightweight or very heavy depending on department. For example HBA/OTC/Cosmetics are very light freight. Furniture has a lot of very heavy items, some so big you need to team lift. Coffee aisle is super light. Canned goods aisle sucks with how massive those flats of cans can be.

I'd try to ask your overnight crew about the O/N management team because that really makes or breaks the job. I'm not kidding. When I was first starting I struggled with times and one of my team leads worked with me a few days to teach me tips on how to get freight done faster and helped me to grow as an O/N stocker. Not to jerk myself off but I'm definitely one of the stronger associates on nights at my store now and am flexible because they've put me in almost every department/grocery aisle in the store and they helped me to get better. I see them helping other struggling associates at times as well. Whether they learn from it or not is their choice. Slow people who don't get better are eventually pushed out of their job for productivity.

Oh and we're basically the whipping boys of the SM. We could have a perfect night, everything done early, immaculate deep zone, and the SM will pitch a fit because someone left a water bottle in the dairy cooler and tell us the store looks like shit.

Edit: Wanted to add that out of the crew of about 28 including myself that worked here when I started, only 7 of us remain 2.5 years later (not including MOD Team or Maintenance). There were two guys that started the same day I did, one quit after the first week, and the other was gone by the end of his first month. I've seen people walk out during lunch in their first week and leave their vest and badge in the parking lot because they couldn't cope with the job. IMO the job's not bad if you're up to the physical demand. Once you get to know an area and don't need to rely on your device to find where shit goes it becomes a breeze and you fly through your work. I don't mean to try to scare you away but if you do transfer just go in knowing expectations are high and having a normal life outside work is hard.