What happened to my cake?
Over-rising and then sinking sound very much like a cake made with too much leavening. 

Since you cake is made with the creaming method, and baking soda, there are two sources of leavening:


The baking soda, as a chemical leavener, will react with the acid from the lemon juice and butter milk.  Have you changed brands or amounts of any of these items that might change the acid ratio?
Creaming the sugar with the butter causes the sharp edges of he sugar to cut air bubbles into the solid butter phase, which serve as the seeds of the leavening.  Has anything changed here, such as starting to use a stand mixture?