Problems with my SUV Transmission?

So I took my 2002 Ford Explorer Eddie Bauer 4x4 V8 to the shop to get new transmission fluid because I have drained it by accident because I thought it was the oil drainage plug, anyway I went to have the new transmission fluid put back and I payed $140.00. Later I come to pick it up the mechanic says its starting to slip and my axles are bad because I have had drained the transmission fluid, the transmission was new only had 40,000 miles on it, could he have done something to it by purpose or is it normal that it is slipping because of the transmission fluid?