Sorry, but I don't buy that official story.
I think Cruz was afraid Trump would ensure Cruz would not be re-elected. So he groveled.
What happened was the next day after his convention speech the Texas delegation
"requested" a meeting with Cruz and gave him hell. Most likely told him he would be
primaried (by Dan Patrick?) if he broke his pledge to support the GOP nominee no matter
who that nominee was. Cruz then found a convenient excuse to support Trump. One
of the reason I like Cruz, with enough conservative heat Cruz sees always the light.