I can't say why they were changed specifically in
August, but what really changed was the RNC. In all previously years, the Colorado caucus straw poll had been
non-binding, but the RNC apparently passed a rule saying that such straw polls had to be binding.
That was the change. And it was in response to
that change -- which meant that they could no longer hold a
non-binding straw poll -- that Colorado decided to get rid of the straw poll.
So the bottom line is that Colorado chose it's delegates the exact same way they'd been doing for years.
I personally don't like caucus systems, although I understand why parties have them. But there is no indication that Colorado chose to stay with caucuses versus a primary because of Trump.