Just planted a 40 gallon mexican sycamore a few weeks ago.
Dug a nice big hole, planted, back filled with high quality soil, mulched, staked. Water has been via a drip system with occassional supplemental hand watering.
Now seeing a lot of brown leaves.
Just transplant stress? Or should I be worried? I'm in Houston and lost a lot of trees the last two years due to the drought.

You could be overwatering it. Soil needs to be moist, but not waterlogged. Did you leave a pipe in the hole you planted the tree in to pump out the excess water?

No I did not. I'm not familiar with that technique.
I seriously doubt i am overwatering it, i just switched the irrigation from a microsprayer to a micro drip, i also just changed from 20 min every other day to 35 min every day because i didn't think the sprayer was getting enough penetration into the soil.