@beermad is correct, and so is @Teo and @Aragorn - you must regularly do some housekeeping.
It is common knowledge that Manjaro inherits the vast majority of packages from Arch Linux.
When Arch Linux maintainers remove a package from the official repository then don’t just throw it away as there may be users that depends on the package, so as a courtesy (my guessing) towards those users the package script is added to AUR.
The native package manager (pacman) does not use the AUR.
Manjaro users on the other hand uses Pamac GUI for everything, including AUR. This will lead to rebuilding package scripts for packages once in the repository, but moved to AUR.
This fact has lead to topics, intended to help users understand their system, leading to a better update experience.
Keeping several kernel packages, their matching headers and orphan packages (no longer required by other packages, or deprecated (moved to AUR)) lead to extensive downloads and possible dependency hell.