The reason for shemittah might be:
Shmita was given on Har Sinai similar to the midbar where they were free to learn so are we free to learn for the year where we refrain from working the field. Says the Yismach Moshe that shmita is against all the shabbosim that were not properly observed for seven years. A year we have 52 shabbosim with one day left behind so in seven years we have 364 shabbosim (7 x 52) with the day left behind makes it exactly 365 shabbosim.