The increase in pioneers is probably due to the cart witnessing. It's nothing now to spend 3-4 hours in service standing around.
The carts have been around for a while now and I suspect we are seeing that more JW's are getting used to the idea of using them. Those who may have been intimidated by Pioneering in the past because of face to face encounters and the amount of knowledge they might be expected to convey in the door to door work, may see the carts as a more doable alternative. The carts I've seen are typically manned by 2 people and there are usually 2 carts near each other. That's 4 people (possibly all pioneering) and counting time standing in the same spot and being seen by virtually the same passers by.
I've also noticed that there's a lot of older ones doing the cart work and especially and increase in older men who are standing by the carts. I'm getting the idea that the "baby boomers" are retiring and Pioneering with the literature carts, is an easy way to go about it. It may be more affordable for them rather than driving a vehicle around all day.