Video's worth watching, but the gist of it: a free browser extension promoted by YouTubers and owned by PayPal is advertised as a way to very easily apply the best available coupon code to web stores at checkout. It often fails to find codes, but it also adds a PayPal referral code/cookie to your purchase, so they net a referral fee, even if it found no coupon.
This is nefarious in and of itself, but it also means that the channels they sponsor lose out on referral fees from their viewers, because it replaces their own referral code/cookie. YouTubers would not have known this when accepting the sponsorship and promoting the browser extension.