I normally jump right into the middle of a webcomic, so what usually forces me to jump back out is...I guess I'd call it 'too little information'.
I don't need a 'Previously On' segment with every chapter; I don't think it's fair to require authors to backtrack just so that I don't have to go back and read the comic myself. But, I mean...I dunno, I just feel too lost when I can't even figure out what the stakes are on my own.
There's nothing inherently wrong with having pages and pages of characters just talking or just fighting, but when I can't even tell why they're there or what their relationship is or what all this talking/fighting is leading up to, I usually just leave. I give 'em ~20 pages, and if after that I'm still not interested, I'm out.
A recent example involved this comic that actually looked pretty interesting; there was fighting, and characters were even dying when I got there: but despite all the action, I received so little plot-relevant info that it was difficult to get invested.
What's more, the author would say things in the description that you'd think their readers would know already, like 'Character A said x when Character B died as a reference to their deafness'. And I was like, if this character has been in the story for 8 chapters already, why do you need to 'explain the joke' to your audience, so to speak?? It made me feel like it was entirely possible that the author just wasn't very good at telling things to the reader in-story, and that I wasn't going to feel any less confused anytime soon...