A. This effect is perceptual/psychological. The hummers are unconsciously increasing their own volume. It stems in part from the innate difficulty of controlling the volume of your voice. Because your voice conducts very strongly through your bones and your mouth is generally much closer to your ears than anyone else's, your voice generally sounds very loud in your head. Over time as children, we learn to control the volume we're talking at by knowing instinctively how loud we need to sound to ourselves to achieve the required result for others. Thus we can generally make ourselves heard over background noise or a reasonable distance with relative ease.
This instinct fails if our ability to compare our voice with other sounds is affected. For instance, those loosing their hearing often unconsciously talk louder in order to increase the volume they hear themselves at to the level they're comfortable with.
In the case of the singing or humming group, several factors are conspiring to make it difficult to maintain a constant volume. Firstly, as everyone is making the same sound, it becomes difficult to tell exactly which sounds are your voice and which are others. Secondly, this large group of people who are each individually singing at the same volume as you has just moved closer to you - increasing the volume you get from them. If you all hold hands, then sound can conduct through your bodies as well, further increasing how loud the others around you sound. These two effects through off your perception, which usually comes to the conclusion that your own volume has dropped and thus you begin to hum louder. Once a few people are humming louder, then the volume really is higher, and most of the rest of the group will catch up. This effect loops over and over increasing the volume of the group until it reaches a point where there is either no more volume available to the participants, other physical effects like throat pain alert the hummers to their volume increase or the increase reaches a point where, due to the non-linear response of the ear, your perceptions don't notice the volume increases around you anymore.
In physics, (and many other fields) this sort of effect, where the increase in a quantity (in this case volume) causes processes that further increase that quantity (humming louder), leading to more of the same processes etc. etc. is known as positive feedback. The reverse effect - negative feedback, where the increase in a quantity causes processes that reduce the quantity - is also common.