5. Justin Herbert having a sophomore slump
While this is probably the most unlikely of the bunch, it certainly is possible and has happened before. There is a reason why there is a name for it already as other quarterbacks in the past have had a sophomore slump.
There are two things that can happen. Justin Herbert could either get even better at reading defenses and with better resources can take a step up in year two. That is the more likely of the two outcomes.
Or, a new offense mixed with teams having more tape on him to analyze and figure out his tendencies can lead to a tougher second season than his first season. That is less likely, but certainly possible and has happened to notable quarterbacks.
Baker Mayfield had a worse season in 2019 than he did in 2018, Cam Newton took a small step down in his second year and you could even argue that Andrew Luck regressed ever so slightly in his second year.
Typically, when you get to the level that Herbert is at there is no going back. There is a threshold that if you cross you cross forever and Herbert seemed to have crossed it, but who knows, nothing is guaranteed in the NFL and he has never had to play against a sold-out home crowd while he is on the road.
We expect Justin Herbert to break records in 2021 but who knows, he could end up surprising us for the wrong reasons.