The rise of Sanders and Trump (i.e. political outsiders) has coincided with the rise of self-described "independents".

Over the last six years, Democratic identification has fallen from 36% -- the highest in the last 25 years -- to 30%. Meanwhile, Republican identification is down from 28% in 2008 to 26% last year.
These changes have left both parties at or near low points in the percentage who identify themselves as core supporters of the party.
...
Since 2008, Americans have been increasingly reluctant to identify with either the Republican or Democratic Party, and now a record 43% claimed political independence in 2014. Given historical trends, 2015 could bring a new record, as the percentage identifying as independents typically increases in the year before a presidential election, averaging a 2.5-point increase in the last six such years.

50% of voters believe that neither party is the party of the people. What's more, most believe their own representatives have sold their vote and think congressional elections are rigged to benefit the incumbent.

Only 16% don’t think most members of Congress would sell their vote, and 25% are undecided. Twenty-eight percent (28%) don’t think it’s likely their own representative has sold his or her vote, but only five percent (5%) say it's Not At All Likely. Sixteen percent (16%) are not sure.

Of the independents, a slight majority say they "lean" Republican, which sort of makes sense since Republicans have suffered the most from this decline.

The media and political pundits are at a loss to explain the strength of these two candidates, but that's only because they aren't listening to the voters. The reason for this trend is obvious.

Both the trend to independents and the trend to believing the government is hopelessly corrupt started around the 2008 financial crisis and gained traction after the Obama Justice Department decided not to prosecute anyone for it.
This isn't an interesting anomaly. It's a full-scale, loss-of-faith in the system and democracy, political crisis.

In this context, it sort of makes sense.
Donald Trump is "too rich to buy", while Sanders refuses a SuperPAC and is running on an anti-Wall Street/pro-Single Payer platform.

Those ways of looking at it have flaws.
While it is true that Trump is "too rich to buy", the narcissistic bully that is The Donald isn't "too rich to sell you out". Not by a long shot.

As for Sanders, he is speaking and doing the things you would expect from a man of principle, but then Obama promised us "change".

Given the overwhelming percentages of Americans who believe that the government is corrupt, you would think that someone would create an anti-corruption platform to run on similar to the Progressive Movement of the early 20th Century.
But since neither party has done so, it just reinforces the widespread belief that Congress is out of touch.

It's important to acknowledge this trend change because it means that something that has marginally worked since 1992 won't work anymore.
Since Bill Clinton the Democratic tactic has been to tack right and fill the political center as Republicans move further and further to the extreme right. Since Hillary is closer to the center than Sanders, it would only make sense that she would be more electable.
At least that is how Democrats see it.

The unpromising lead is: Sanders polls 30% behind Clinton.
This bad news might be best explained by the Democrats’ even more lopsided answer to the big “electability” question. Unfortunately for Sanders, 38% more Democrats think Clinton “would have a good chance of defeating the Republican nominee” than would Sanders (87% to 49%).

That makes sense based on what sort of worked from 1992 to 2008.
However, it doesn't explain this.

The good news is what the pollsters actually demonstrate to be true about electability by direct match-ups of the two Democrats against those four Republican contenders who have more than single digit support. Their findings: “Sanders does just as well [as Clinton against Rubio], or even better, against [the other] top Republicans [Trump, Carson,and Cruz].” Against each of the latter three, Sanders’ winning margin exceeds Clinton’s by 2%, 3% and 5% respectively.
It appears that democratic voters are not just misinformed, but grossly misinformed, about whether Clinton or Sanders would do better against Republicans.

Being grossly misinformed is what will look like when the political environment changes and you don't sense it.

There is a real danger involved here when it appears that a sizable percentage of Democrats willing to vote for Trump rather than Hillary.
Which doesn't make sense if you are still using the center-right political strategy of the 1990's, but it does make sense if you use the establishment vs. anti-establishment spectrum today.