What would be the reasons for the USD to fail? The only one I could think of is that the Union would disintegrate, Balkanizing the country, but why would that happen? You know the Euro since it's beginnings was given only a year before it would collapse and since then every year someone says this is the year the Euro fails and yet it's still working.
Oh man, it's a shame that you don't know the reasons why we're using bitcoin and yet you're still on this forum.
The exact reasons of using bitcoin in to hedge against such a devastating event, such as in case the USD fails. This could be brought about by any sort of bubble that is in the US economy, for example the 2008 housing bubble came real close to wrecking the whole financial system. After all, the government is in charge of the economy, and they are going to do wrong things.
e.g. after 2008 the government was in a pretty desperate situation to make the economy function correctly again. They decided to inflate the currency base, thus reducing the value of the dollar you hold in your wallet in the long run. It mightn ot reflect immediately due to sticky wages but it will reflect eventually.