My basic understanding of snubbing is that it is used (as you say) when there is excessive pressure in the well (live well). The pressure acts on the tubing cross sectional area and pushes it out of the hole. The snubbing unit can apply force in addition to the weight of the pipe to trip the pipe into the hole.

Just a guess here, but if the well is live because of the first frac, then they would need to snub pipe into the hole to set a bridge plug to block off that zone for now. They could then run in conventionally to perforate the next zone up.

They would choose snubbing over "killing the well" by filling it with heavy fluid, as this would likely cause formation damage.