Maiar - that can be the first sign of cracking on borderline cracking juices, it etches the plastic at the weak points, one of many ways they frost glass with a powerful acid. Stronger PH liquids, those same weak spots in the plastic, the liquid reacts more violently while it seeps in, causing heat and expansion in the plastic, similar effect of how ice cracks a stone only with heat. Thoug it is not a noticable amount of heat generated you can feel to the touch, it is enough to generate force on the microscopic level.