A great series!
The reason people are making these accusations is that you've been posting these on here, while the entire series is already on multiple other sites.
If you say that you're working on something, and then someone finds that what you're working on is already done and can be found elsewhere, it raises suspicion.
That said, I don't think this is plagiarism. You clearly did release this series on other sites before this. That said, it would obviously be unwise to release all three parts at once, as they would probably not be received by viewers in the right order.
To any sceptics on this issue, go check out his page. He has other animations (including another Minecraft one), and it's easy to tell by the art/animation style that it's the same person making them. Furthermore, I'm sure by now that if this was ripped off, the original author would have found out and reported this.