This is the most replete article I've found on the issue:
‘Bernie Or Bust’: Bernie Sanders’ Supporters More Determined Than Ever After Chaos At The Nevada Democratic Convention
Denying delegate status for not registering as Democrat by May 1st is plausible, but that this was the case for 91% of Sanders delegates is definitely not.The “Bernie or Bust” movement has grown now that Bernie Sanders’ supporters are outraged over the chaos at the Nevada Democratic Convention on Saturday. Despite having less support during the convention, Hillary Clinton was awarded more delegates. The decision caused Bernie supporters to become outraged as they believe the Nevada voters are victims of election suppression.
It turns out that Bernie Sanders actually had more support at the convention than Hillary Clinton. However, the convention did not count 58 delegates for Sanders because they said they did not switch their party affiliation to Democrat in time for the convention. The delegates had until May 1 to become Democrats, but at the Nevada Democratic Convention, 58 out of the 64 contested delegates for Sanders could not be found as registered and were stricken from the count. Due to that issue, Hillary Clinton was able to take Nevada by 33 votes.
Sanders’ supporters are angry because they claim the convention rules were changed literally at the convention in order to purposefully deny certain delegates from voting in order to ensure that Clinton took the majority of delegates. Then, when the underdogs demanded a recount of the delegates because they believed they were being cheated, that request was denied as well.