This May I planted 25 arborvitaes about 5-6 ft tall. All looked good for a couple months but about half have started to brown and die starting with individual main branches. I water 3 to 4 times a week and have mulch beds around them. Some have been healthy all summer and have just started to brown like the others. I find it odd that the browning starts on main branches then slowly spreads. Any advice?

Here in N. Illinois with heavy soils, watering 3-4 times a week would have likely been causing lots of root rot. If you have very sandy soils that drain well, that amount of water may have been appropriate. Depending on how much water you provided each time coupled with a water conserving mulch they could still have been overwatered. Clear away the mulch and if the soil is really moist/wet, I suspect that is the problem. Whole branches sectioning out is not uncommon when root and crown rots are present. Container grown plants could also be suffering from circling roots if not attended to a planting time. Undersized balled and burlap plants can also be a big factor. Depending on what you find, hold off further watering if the soil is very wet and let some soil oxygen get in there and dry out the profile. Roots need oxygen to absorb nutrition and water.