This is specific to webcomics, because most professionally edited comics I've read in recent years seem to avoid it (there could be exceptions, I don't read EVERY comic) but I'll pretty much stop reading the comic if it starts out with a huge block of exposition. This kind of opening present in a lot of video games, especially JRPG style, and I think that's where the influence to webcomics comes from.
For example, the opening where a thousand years ago some stuff happened. Or, main character narrating how boring their life has been up until this point. Or, some really interesting stuff happened, but 20 years ago and we only told you that story summarily so we could tell you this much more boring one that takes place later. Or, the opening is a forward flash to when something interesting will be happening, but they have to back up and tell us this long boring story, just to bring us back up to that point where we initially got invested.
To me, that kind of writing shows a lack of confidence on the writer's part to convey the story to readers without telling them specifically. Specifically jumping back and forth in time usually (not always) strikes me as a way for the author to cover a weakness they feel they have. Like, I know the story is too boring to hook people immediately, so I just jumped to the good part and then went back. These tricks can have constructive uses to creating a story with texture and mystery as well, but they're more often used sort of like water wings. Useful, but can prevent you from learning how to swim.
