In the cases of TAG and TC39, I saw an opportunity to take low-hype, low-bandwagon organizations and revitalize their missions and purpose. In particular, bringing on web developers as active participants and the follow-on effects of having them join GitHub and modern practices (slowly), helped increase their profile and stature. In both cases, I hardly did most of the work, but I did spend a lot of time articulating a vision for these organizations as ones that could be far more effective by involving more practitioners. I think it has worked.
In the cases of Ember and Handlebars, I saw something missing in the ecosystem and built my own tools. In both cases, the tools were hardly instant-winners, and I had to spend a ton of time recruiting fellow-minded collaborators who shared a vision for the future. If I was in it for the bandwagon-hopping, I would have tried to join Mustache or Angular, and not spend years to build up my own, relatively small-in-comparison ecosystems.
My real MO is to try to envision a better future for something related to the web or my product, and then either find existing projects that already share a part of that vision or create them if they do not. My involvement in many different projects is because of the fact that big-picture ideas involve improvements to multiple technologies.
In this case, I think you've got the causality mixed up. Projects often gain prominence thanks to the work Yehuda puts into them, as he's particularly good at articulating their strengths to a wider audience. It's not that he's bandwagon hopping—it's that he's helping to create the bandwagon. ;)
Likewise, it is illogical for faulting a developer for possessing that same drive when it comes to improving emerging technologies.
Squandering one's skills to appease some anti-progress group is silly.
It would probably be much easier for them to stick to one thing but that wouldn't fully utilize them as a resource.
Furthermore, if you continue to bring something to the bandwagon, people will continue asking you to join them.
Whether this perception is right or wrong, there are a number of prominent individuals within the open source community who are widely seen as rapidly jumping between trendy or hyped projects as they arise, to benefit from the exposure that this involvement can bring.
I'm not passing any judgment, mind you. I personally don't care which projects you choose to get involved with. However, I do know that other people have noticed certain trends around how certain open source developers move among projects, and this does negatively affect the impressions these people have of these open source developers and the projects they get involved with.