There is some history for a lot of people, though this is a simplification and not applicable to everyone.Essentially, a lot of people are "against Establishment Politicians". What that actually means, few could tell you. But it means people want someone "outside of the machine". It's how Cruz got elected senator to begin with and why Trump is doing so well.
Sanders is the left version of Trump. Some on the left felt that current democrats aren't liberal enough (I.e. Instead of universal healthcare, we ended up with the ACA). So they tried to get Elizabeth Warren to run, and when she didn't the germ of the Sanders movement was born.
In a nutshell, people who support Trump and Sanders have a few common qualities: they are unhappy with the Establishment and want to radically change it. The difference is the direction of that change and what actually constitutes the Establishment.