The big cause among Clinton supporters right now seems to be that her name is formally submitted for nomination at the convention and that a full vote is held. See this press release by Clinton Democrats for more. Some of the comments there are very telling. For example:
If Hillary’s name is not on the ballot in Denver, I will make sure that I am no longer a registered Democrat and will support the Republican nominee.I am really curious how or whether this gets resolved in the next two weeks.