Why did Andrew Jackson and his supporters consider the election of 124 to be a "corrupt bargain"

It was considered to be corrupt bargain because Andrew Jackson removed civil or federal workers and replaced with his supporters during the election.

You must mean election of 1824, well to keep it short John Quincy Adams was friends with Henry Clay who was speaker of the house which many thought that he struck a deal with him to persuade the Congress in voting for Adams. Hence the "Corrupt Bargain"