Some mods change the fov for things like zooming in, however it is impossible to set it back to the player's choice of fov, so it just sets it to the default 90.
You can either:
Press F after it resets it every time.
Slowly get use to 90 fov.
Disable/Fix add-ons that are doing this
Some add-ons, like gravity cat's default weapons pack come with a /fixfov command which allows the user to enter in there own fov so it resets it back to theirs instead of 90.