Why might a cake collapse after cooking
It's possible that your new brand of self raising flower has more raising agent. This causes a initial raise to happen faster resulting in large 'bubbles' that are less stable. The cake rises higher and then is more prone to falls.

Fast changes in temperature or air pressure will cause the cake to collapse. That means every time you open the oven, or if your oven isn't very air tight every time you open an close the kitchen door. Loud slams of doors are especially bad.

Higher levels of raising agent are especially an issue if you live at a higher elevation or if there is low air pressure that day.