If you bought them from a local PetSmart you likely have the same water parameters as the store so itís unlikely to be ph shock.
Weíd need way more information to have a chance of knowing what caused the problem in this case. Thereís just too many possibilities. If you want to get to the bottom of this answer the questions in this thread so we can look for potential issues.
Angels can be really temperamental on water changes I never add fish or add water with out stress coat especially if you use tab water and if you run a ph test on the water they strive I believe between a 6.8 and 6.6 on the test kit and if you did not let them sit in water in bag for 10 or so minutes the water temp can put them in shock if they are bullied when added to tank they usual dont survive that as well