I found rstudio is loading a package automatically in project startup, when I didn't save/load workspace. Turned out that I have some tabs of data/variable viewer open, which needed the package.
I think with previous version of rstudio, reopen a project will just show those data viewer as data not available since they are not in environment anymore. But somehow version Build 832 (a17fc948) will load the needed package.
Since the workspace was not saved/loaded and user probably want a clean start, I think the proper behavior is just discard those data viewers as before, not loading package and try to show them.