In some cases, violent behavior has led to criminal charges—in Newark, N.J., for instance, where parents allegedly beat up a Little League baseball umpire because he wouldn't call a game because of darkness.
"The level of competition in youth sports has gotten exponentially greater, forcing this level of hyper-competition," Sanders says.
"I think that is driving a certain level of behavior on the sidelines that is amplified."
Haley Small, a 19-year-old college student who played soccer and then traveling softball through high school, puts it this way: "The more competitively I played, the more interesting the parents got."
"We'd joke about it, but it's serious. Some of my friends were walking on eggshells," says Small, now a student at Ithaca College in New York. "We hear a lot more than people think."
It gets so bad sometimes that some players wish their parents would just stay home, she says.
Laura Marinelli, who coaches Small's younger sister on a traveling softball team for 12- to 14-year-old girls in Essex County, N.J., also has noticed more over-the-top parent behavior in recent years.
Marinelli recalls one dad who was angry about a play on the field and tried to tackle her assistant coach during a game. The coach was able to duck the parent and ended up throwing him to the ground.
At a national tournament last year, she says a father of a player was so unhappy with a decision she'd made that he ran at her in the dugout, screaming and pointing in her face, causing some of her players to cry. Ultimately, she asked his daughter to leave the team because she felt the dad had repeatedly violated the team's code of conduct.
"The girl is a phenomenal softball player. She's a sweetheart—and a great kid," Marinelli says. "But I can't have a parent like that on the sidelines."
Kicking kids off teams is one of the more serious punishments that leagues and coaches use to try to keep parents under control. Some leagues and tournament officials also are giving umpires more power to warn offending parents and coaches and then ask them to leave the premises if they ignore the warning.
It can be an effective deterrent, though in many other instances, umpires or referees at youth games are often teenagers who may not have the experience or confidence to stand up to parents.
And often, there's no security at games. So parents are left to police themselves.
For that reason, some teams assign parents to be "culture keepers," asking those people to help keep the yelling and negativity from fellow parents to a minimum. Sometimes, they even hand out lollipops to help keep themselves quiet.
"But sometimes the culture keeper isn't always the best person—because that person is yelling just as much as the other parents," Jill Kirby says, laughing. She's a mom in Long Grove, Ill., whose five children participate in sports, from soccer to swimming and T-ball, sometimes in neighboring Buffalo Grove.
She says the signs asking adults to behave are a nice idea - perhaps even a way to get people talking about the issue. But ultimately, she doesn't think the tactic will work.
"I think the worst offenders don't think they are the worst offenders," Kirby says, conceding that maybe even she was one of those parents, "once upon a time."
"And then I got a little perspective," she says.
Greg Dale, a sports psychologist at Duke University, agrees that it's difficult for parents to see themselves as "that parent," at least without a little help.
He recalls a mom in California telling him about a dad she called "leather lungs" because he yelled so often at the officials, coaches and kids.
Hesitant to approach him, the woman secretly filmed him at several games and anonymously sent him the video. "And the guy changed the way he was acting from then on," Dale says.
More often, though, he says he sees parents who "say the right things" about sportsmanship—maybe even reciting a pledge before a game, as is the case at his own children's Little League games.
"Those things help. But ultimately, I think they're Band-Aids," says Dale, author of the book "The Fulfilling Ride: A Parent's Guide to Helping Athletes Have a Successful Sport Experience."