Trump was at 1 to 2% in June, and is leading the polls today, less than 3 months later. What makes you think others can’t do the same in 5 months?
Maybe because Trump only announced three months ago. Not one other candidate has come from lower digits to even consistent double digits except Carson. He, like Trump, is an outsider---but he has been able to cruise along in Trump's shadow without answering hard questions. But Carson has not yet led in any poll in any state, while Trump has led in all the polls save two in every state.
Fiorina gained far more than Cruz---but has yet to crack double digits.
In other words, the minor churning in the lower tier is meaningless.
So, no, Cruz is not going to leap from 4% to 20%.