Did Emma mention that potter's trick with the extra coil? I've tried it and it really does help, but you have to be careful not to add too much or it makes the wall uneven. I've found wetting the area right at that 6 inch spot before you pull up the next section can make a huge difference too. It lets the clay stretch more instead of just cracking under the stress. Stiff clay is a killer for tall pieces, it just doesn't want to move.
Honestly that wetting trick at the 6 inch mark is solid advice. Ngl I've ruined so many pieces by trying to pull up stiff clay without prepping it first. Tbh the extra coil thing helps but you gotta blend it in real smooth or it just looks lumpy. I learned that the hard way.
That 6 inch mark is a real stress point on a lot of wheels. I saw a potter at the local studio who always reinforces the inside curve right there with an extra coil when she's throwing anything tall. Maybe the clay was just a little too stiff and couldn't handle the outward pressure?