It's much like how Porsche cornered the market in Volkswagon shares. Until the last couple hours, the market at large had no idea how close they were to a corner.
Also, remember that the price of a stock is set at the margin, but the value of one's holdings is an average. Imagine that an investor has amassed 100M shares of Google at an average price of $20. If they get to 100,001K, the value of Google will shoot up to $50. It is rational for them to pay any price up to $3M for those last 1000 shares, because their profit if they get them all is $3B, but if they are missing even one of them, they get nothing.
Same reason eminent domain laws exist: without them, it is rational for a large organization to pay any amount up to the total value of the project to the last few holdouts that are preventing it from being completed.
If there is a big difference in price then that's an arbitrage opportunity for somebody, in the end all shares have the same claim on the company and will get the same payout if Google is bought or split up. In this case there would be a bunch of people buying class C shares in anticipation of the activist shareholders winning. Additionally, the price of the voting shares won't go up that much because the activist investors can't pay more than what they perceive the value of the company to be.