In the original Planet of the Apes, the reason for their travel forward in time is their near light speed travel, which causes time to move slower for them. This is scientifically accurate, allows them to know the Earth year through calculations, and seems to be abandoned later on.
Suddenly in the sequels, they encountered some sort of vortex that pushed them forward in time, and allows travel in the opposite direction.
Why would they hastily abandon what had been established in the original? Do the sequels remove the original reason for their jump ahead in time?