Short Answer: Frogs. They even managed to become the 2010 World Champion Deck.
Longer Answer: It was banned because of its
relatively easy abuse on OTK and FTK Burn decks, that focused on continuously summoning frogs to sacrifice them and drain your opponent's LP.
Basically it consisted on "dumping" as many frogs as you could to your graveyard with the help of Substitoad, so you could then bring Ronintoadin to the field easily with them, and Mass Drive the f*** out of your opponent.
The key parts of the combo were:
- Have Substitoad and another monster to tribute on the field
- Tribute that other monster to summon a frog from your deck, with the effect of Substitoad
- Repeat step 2 with that summoned frog, untill all of them (at least 18 or 20 as you correctly pointed out) are in your graveyard.
- You can then bring Ronintoading to the filed as many times as you want (removing a frog from your GV each time), then use Mass Driver with it until you win.
As you can see this was a really broken combo. It was even supported by cards like One for One, Double Summon (to put Substitoad and another monster on the same turn), etc., which were other reasons why this Deck and its variants became too strong, hence the main reason for banning of Mass Driver (Substitoad was also banned FYI).
We can see that in the current format, this card would still be quite strong, as you could well do 2000 LP damage if you Pendulum Summon 5 monsters and then tribute them. You can even tribute tokens. You can further combo it with cards like Dark Room of Nightmare, in which case it would even take less than 20 monsters tributed to win the game.
Edit: Another posible abuse that occurred me (in a more general sense) is that it enables you to "just" tribute a monster when you please and the times you please. Any "when sent to the graveyard..." effect can be triggered this way, as well as any "when this card is tributed...".
It would be a shame if your opponent takes control of a monster of yours, and after using his effects/stats against you, tributing it for another -400 while preventing you form getting it back.
Also, good bye any pesky Ojama Trio tokens, Flying "C" preventing your XYZ, or any other monster card you have that don't need on the field. You can also turn a Dandylion into 1200 Burn damage. As well as one of my favorites: Reborn Tengu.
This could also be helpful to, say, Hieratic Dragon decks. Another really broken example I can think of is to use it with a Qliphort Deck, which heavily benefits from tributing like Qliphort Carrier, and happens to follow a Pendulum strategy (swarm, Mass Drive, Qli effects + Damage,...,GG).
Any other you can come up with? I am sure we can think of many more possible uses. So definitely a card that could unbalance the game if left unrestricted.