For the last month or so, the Yosemite Falls has been basically dry, which is normal during the summer. Lately, however, there have been some significant thunderstorms in the sierra, and now, lo and behold, the falls are back for awhile.
Here, the high country is all granite, and there is mighty little soil to hold the water back. When it rains, or the snow melts, it all basically comes off in a torrent, and then it's gone.
If the thunderstorms abate, then the falls will peter out and die until another blast of water comes in from the Pacific and rains out on the peaks.
Enjoy the flow for now. It might be gone in a week.