I don't think there's been a rise in the minimi usage, but the ones using it have definitely gotten better. Such oponents are so much more effective these days. I tend to use it as well, since it's amazingly good at holding the other team at bay(as said above).
As for the minigun, the way I see it, it's a fun-weapon. People using it more and more is just a sign of being more relaxed about the hole soldat thing, instead of being so focused on simply raising your score. It's a mental thing, really. Diving deep into the human psyche allows us to understand the pattern developing when playing impulsive, yet simple games such as soldat. Somehow along our quest for the highest rank, we tend to get bored with just killing.. Now we suddently feel drawn to the "moments". What I mean by that, is instead of being number one all the time, you wanna live out the moments where you feel like, "Woah, 4 vs. me, and i nailed'em". The moments where people go like, "w00t, HAX!"... The minigun is kinda hard to use, so being godlike with this particular somehow spreads the word: "You might as well disconnect"
Or the mimigun being used more could just be a coincidence.
