This Friday was the deadline for a big Computer Vision conference (CVPR), so effectively you're seeing what everyone has been working on over the last half a year :) Some authors choose to publish right away to arxiv, which is why you're seeing this influx right now. Some authors choose to upload to arxiv later, and some choose to never upload and wait for the whole review process to finish in some number of (long) months. I think the community is slowly deciding that the field and the ideas evolve too fast, much faster than the length of the review process. For example, I'm about to present my work from March at NIPS, happening in a few weeks. I've almost forgotten what I did, and I deprecated my NIPS model 3-4 times. It's not a chance to present cutting edge research, it's a chance to talk to your friends/colleagues and stand next to your poster awkwardly, trying to sell a model that you now know doesn't work very well.

Ah, no wonder, thanks! It still seems like quite the coincidence that so many groups were independently (?) working on describing images with CNNs hooked up to RNNs, though. I guess it's just an idea whose time has come?