Normally 45 auto cases have a really, really long service life. I saw one test where they stopped at 45 or 50 reloads on some cases because the head was getting visibly deformed, otherwise the brass was still reloading and functioning perfectly. Sometimes bad cases happen.
Some other causes can be an out of spec sizing die. According to the SAMMI drawing, a 45 acp body diameter should be .473" - .006" tolerance. So if your sizing die is sizing your cases under .467", then your die is out of spec and overworking your brass. Another cause is probably not the case, but soaking your brass in something such as vinegar can weaken them. Usually a quick wash doesn't hurt, but if you are soaking them for say 30+ minutes, that is not helping.
More than likely you just got unlucky.