So, in the episode, Trent explained to Gwen that the reason he's afraid of mimes is because when he was 4, he got lost at a carnival and was traumatized by a mime, and later in the episode Courtney's aversion to green jelly is implied by Gwen's taunting to be because it resembles mucus. Those revelations got me thinking about some of the others' fears, specifically some of the more unconventional ones that are never elaborated on in the episode like Harold's fear of ninjas, Heather's fear of sumo wrestlers, Tyler's fear of chickens, and Duncan's fear of Céline Dion music store standees. Here's my specific headcanon for those four:

• Heather: this is a fairly widespread one but it deserves a mention here anyway - Harold stated in Total Drama Drama Drama Drama Island that he thinks Heather's such an insufferable bitch to everyone as a defensive behavior, which he explains as "either your parents are divorced or you were really fat and pimply once." There was a series of shorts made when there were 5 campers left, and Heather's shows her parents partying because of her not being home all summer, which means if Harold was right at all, Heather was once really fat, of which sumo wrestlers would remind her.

• Harold: stated through one of Gwen's confessionals that he's afraid of ninjas, but he takes them on as best he can in the challenge. Personally, I think Harold's specifically afraid of being attacked by ninjas, stemming from what was probably a really young Harold watching a ninja movie and being freaked out about the chance that ninjas could be anywhere and ready to attack him.

• Tyler: same specifics as Harold's - he's afraid of being attacked by chickens, likely due to some unspecified childhood trauma where he was attacked by a chicken

• Duncan: this one is so unconventional that it could be played for laughs, however I have an incredibly dark headcanon for it - when Duncan was a kid, he was a witness to an armed robbery at a music store that just so happened to have a Céline Dion standee, and as a result he can't help but think about it whenever he sees one

Everyone else's are either incredibly common fears (like Owen and Izzy's fear of flying, LeShawna's fear of spiders, DJ's fear of snakes, etc.) or incredibly justified (Gwen's fear of being buried alive, Cody's fear of defusing a bomb under pressure, Sadie & Lindsay's fears of bad haircuts (fits their characters well since they care so much about their appearances), etc.)