If Mr. Trump does not have 1237 bound delegates at the beginning of the convention, Senator Cruz will quite likely win on the "first ballot". Senator Cruz knows this as does Mr. Trump. The belated recognition of this unpleasant fact is the reason the Trump camp is so worked up.
And if Cruz steals the election, where do you think all those new Trump voters are going to go? Not to someone who managed to steal their votes from them.