I was surprised also. I feel that the confrontation that Cruz had with the “protesters” holding Trump signs and yelling “Do the math!” might have pushed him over the edge. It seemed like the pinnacle of a very bad couple of weeks for him.
Priebus continues to make veiled threats. The RNC leadership is posturing to hold onto what little power they think that they have over Trump. No matter what people think of him, Trump has a very fine tuned ability to both read people and a fine tuned perception of who actually has the power. Hillary and the Democrats most likely will not be able to figure out how to sink him, but he will most likely send the SS Clinton to the bottom.
[The following is purely my conjecture, it may not bear any truth to reality.]
I imagine Priebus and Cruz had an understanding, that Cruz won’t need to have any worries running out of money, RNC would continue support him (or Kasich) all the way to the openly contested convention.
At which point (first ballot nobody wins the 1237 delegates), the party leaders can insert any one of their choice to stuff down republican voters’ throats.
At least that was the plan from Priebus’ point of view. Hence he was really dismayed “and surprised” that Cruz dropped out early. Followed by Kasich soon afterwards.
Now the chairman is seen scrambling to save his own bacon and save face for his leadership team.