As far as I've witnessed over the past few months, it's largely manufactured, if you go by the definition that Bernie supporters are comprised mainly of young, white, college boys who are rebelling against 3rd wave feminism by saying nasty things to women. (If you look at a picture of Bernie's rallies, particularly ones that show entire rooms and not just Sanders and the people behind him, the idea that this demographic is true is quickly proven false. There are a LOT of women at his rallies, and age is all over the spectrum.)
That's not to say all self-proclaimed Sanders supporters are angels. There are some Bernie supporters online who go overboard. I have seen in this sub remarks about Hillary that do cross the line into sexism...although they tend to be subtle. Yesterday I read a post where someone used the term "Hillary Hags". I downvoted it, went elsewhere, then saw later they had edited it to a different term when I revisited the thread to read new comments.
You'll also see people talking about her "shrill" voice. Problem is, women naturally have a higher voice. It just can't be helped, it's like the shape of your nose or color of your eyes, and pointing it out has no place (in my eyes) in politics (particularly for supporters of Bernie, a man who has great integrity.) Remarking that a female politician doesn't fit some standard of beauty is something that happens very often to any woman who is in a position of power, popularity, or influence, and so it strikes a sour tone among feminists.
I also see younger supporters who are naive about racial politics kind of being...over-enthusiastic or over-perplexed when wondering why black people and POC haven't already flocked to Bernie. Or being really flat-footed in trying to figure out how to "attract blacks" to the movement. (As if black people don't read this sub!) I'm white, so I view it as a bunch of over-enthusiastic puppies being tone-deaf but potentially teachable, but some non-white folk are upset when they encounter this, because they SO OFTEN encounter really naive outlooks like this and it gets old.
BUT...none of these sporadic missteps I encounter are UNIQUE to Bernie's supporters. There are bad seeds among Hillary supporters (some of who just insulted other women themselves by implying they were chasing boys instead of thinking with their brains), and there are trolls among Trump supporters, among Rand Paul supporters--everyone. EVERY large movement, whether it is political, or some fandom, or some sports event, is going to have its share of bad apples. It's the nature of large groups of people. Given this is about politics in particular, there's certainly a share of gadflies and concern trolls too, trying to change how a movement looks to the mainstream from the inside by inciting trouble, although it's impossible to tell how many bad apples are conscious trolls TRYING to make supporters look bad, and which genuinely think they're supporting Sanders with their behavior (or think their behavior doesn't matter), and conjecture there will quickly fall down a rabbit hole of stupid/conspiracy theories.
Anyone who has been a part of the internet knows not to feed the trolls. But it's looking like Hillary thinks she can control them, to an extent. So what's seemingly happening is that Hillary's side and the media are sifting through all the grains of sand online, and carefully picking out the trolls who are shitposting, and trying to form a narrative around that, called "Bernie Bros", to discredit the movement.
Why would she do this? Because there's a segment of people in the US who have a strong urge not to be slapped with a label they think is unsavory. For example, I've seen people who are a fan of various TV shows or games or other activities, but refuse to participate in fandom, not because they don't love the shows, but because they are terrified of being thought of some crazy Dr. Who fan, or those disgusting people who write fanfic on the web. Or, you know, people are terrified that wearing some outfit or liking some band will make them the DREADED HIPSTER (oooOOoo!). Or whatever. These types of fears extend into politics. Some people just want to be "on the right side". So Hillary's team is crafting a narrative that Sanders supporters are something distasteful that an upstanding person shouldn't want to be a part of, because it'll socially mark them as a weirdo, or a creep, or someone who hates women.