I just stuccoed my backyard bbq island. I used La Habra stucco over
wonderboard, mixed to mfg directions. Put 1 coat on about 3/8" thick.
weather about 80F, no direct sun. Now, about 2 hours after installing
there are numerous hairline cracks all over. What can be done?

Thats the nature of stucco. Let it sit for a couple of weeks to cure and
finish cracking, then put another skincoat or texture coat over top.