Just re-watched the episode with subs and did anyone else feel a bit sad when jessie said "that pikachu...is truly a special pokemon"? Especially when they did that team rocket montage afterwards.
Uhhh it is kinda obvious. He was hit by that legendary attack which powered him up. Might also have to do with him being Zekrom's attack who might of had a reverse effect due to different charges in electric types.I guess the writers didn't want to explain how Pikachu's Electro Ball was Amped up to 11.