Sounds like a safe bet to me. Sourness is almost always indicative of an infection. Were these bottles reused? I have to imagine there was something left over in these bottles or they weren't cleaned/disinfected as well as the others. Inconsistent infections would indicate something wrong in the bottling process. If the infection happened somewhere else, it would affect everything, rather than select bottles.
Did you check all the bottles? If there are other infected ones, you'll want to refrigerate and/or drink them right away. Infections can ferment significantly lower than yeast, so they can cause bottlebombs.