Reversal energy went to discard instead of kingdra

I was playing a whiscash mill deck featuring kingra with the deep sea king ability . I had a manaphy in the active with a reversal energy on and when it was KO'd by charizard ex the energy went straight to the discard pile. The discard pile was highlighted after as though there was something to interact with and, of course, at that point the energy wasn't going anywhere. The ability worked as intended in other instances (even in that same game), so why not that time?