 Some scholars speculate that these tributes began to be used for public festivals, often including religious ones, and thus gradually came to be seen as associated with the priests.Once again, this is unlikely as the Levites who received "ma'aser rishon" were not priests (called ""kohanim"").The "kohanim" received "terumah" and "terumat ma'aser" from the Leviim, not "ma'aser rishon", as would be thought if they had written the Torah to enhance their own agenda.