What might be causing my inner tubes to get rips behind the valve?

I struggled with this once for a while.

One (duh) obvious thing to double-check. Valves usually have a little knurled and threaded ring. Be sure to remove that ring before inserting the valve through the hole in the wheel, and replace it afterward. Otherwise it's a way to push part of the valve body into the tire. The person writing this learned that the hard way.

Another (duh) obvious thing ... make sure the tire pressure is correct. If you don't have a pressure gauge you should get one. Best move: get a floor pump with a gauge.

Third thing. Get yourself some talcum power, put a bit into the tire and spread it around, by rotating the tire in your hand. It will lubricate the interface between the tire and the tube to let the tube adjust its position within the tire a little bit. That helps the tube from bunching up. Careful: Real talcum, not cornstarch powder. Talcum's a mineral with lubricative properties. And like other minerals that come in dust-tiny sizes (asbestos) you REALLY don't want any in your lungs so be careful.

Handling the tire to spread out the powder doubles as a way to inspect it carefully for sharp stuff and other tire damage.

Check that the rim strip isn’t spinning. It can spin during braking force and carry the tube with it tearing the tube away from the base of the valve.

If it is spinning you need to remove it and replace with a new strip or a few layers of tape of your favourite flavour.

Also check the rim itself, underneath the rim tape. It is possible there's a sharper edge, a burr at a spoke hole or the valve hole. If you find something, try and deburr it with a deburring tool, or a larger drill bit, or a small file, or even a sharp knife can trim aluminium.