i think it is the salivary glands swelling up after a long day stuffing grass! nothing to worry about, as long as you aren't trying to work your horse deep on the bit for a while...
If its out grazing, its grass humour or grass glands. All of mine get it and yes associated with stuffing in grass. MIne seem to get bigger on starvation paddocks, I guess as the head is down all day!