When freezing the lower arch, as opposed to the upper arch, the injection is often a block type, so two nerve pathways are knocked out, the tongue and the one supplying the lower lip.
It could be your perception of the salivary build-up, or water from the handpieces. Some people tolerate this much better than others, but a saliva ejection or suction should hold the numb tongue out of the way, and keep the water form building up.
On the upper arch, the injection is usually an infiltration, so only the area concerned is numbed, the roof of the mouth being left functional, so swallow isn't generally affected from a perception point of view.
That might be where the difference is, you feel like drowning, but it is a small amount of water/saliva, but it feels like more due to the numbness.