In my experience, most blowouts of this kind are due to improper installation of the tube. I've made this mistake myself. If you get the tube pinched under the bead of the tire, then when the tube ruptures, it often takes the sidewall with it, causing a separation of sidewall from bead. It is sometimes difficult to see if you've got the tube pinched. It takes a very careful inspection, checking both sides all the way around by pinching the tire until you can see the rim tape. Even then you sometimes miss it. I like to inflate the tire about 2/3 of the way, then deflate it completely and check again before blowing it up to full pressure.