Yes you indeed had a serious infestation. Once those larvae get a taste for roots they're off to the races. The roots become weaker as the stagnant water suffocates them. Free lunch. Personally I hate all commercial potting soils. There are many threads in this forum dealing with this issue, but if I'm going commercial I will only use the best. Screw cost; you get what you pay for. But in the event that you simply cannot find good soil like Foxfarm, just make sure that you're letting the soil dry out a bit before watering again. At least the top inch or so should be dry, and NO standing water in the lower levels of the rootball. Drainage, drainage, drainage. In nature there is a limitless soil horizon around each plant and water can diffuse away. In a pot things are much more restricted and problematic.