Probably first hit might have been Critical Hit, not the second, because:
Only if Ivysaur has max SpA and Raichu has least SpD, it is possible to hit more than 50%, which is impossible in-game.
Lvl 27 252+ SpA Ivysaur Razor Leaf vs. Lvl 24 0 HP / 0- SpD Raichu: 31-37 (44.2 - 52.8%) -- 22.7% chance to 2HKO
Normally, it would take four hits to KO Raichu (it is possible that Raichu is more EV trained than Ivysaur)
Lvl 27 0 SpA Ivysaur Razor Leaf vs. Lvl 24 0 HP / 0 SpD Raichu: 20-24 (28.5 - 34.2%) -- 1.8% chance to 3HKO
If it is a critical hit, it would be more than 50%.
0 SpA Ivysaur Razor Leaf vs. 0 HP / 0 SpD Raichu on a critical hit: 122-144 (46.7 - 55.1%) -- 69.5% chance to 2HKO
Hope this helps :)
(Lt. Surge's Raichu's lvl is 24)