UNDERwatering.
For years when I was new to this, I read all this BS talk of "root rot" and people saying you HAVE TO get that new tree out of the nursery soil and into inorganic soils.., and until you understand the proper care, it leaves you scared to even WATER a tree! Go to ANY thread about a tree that is unhealthy and read the responses, there is always at LEAST one person implying over watering is the issue when it pretty much NEVER is. It takes a LOT to over water a tree to the point of causing root rot and typically it is not even caused simply by over watering! It is caused by STAGNANT WATER. Which, ironically is the result of poor drainage and swampy soil that doesn't dry out and is NOT WATERED FREQUENTLY TO FLUSH OLD AIR OUT. so, make sure you have good drainage in the pot and whether using organic or inorganic soil, use free draining, "open" components, then water DAILY (some trees need it 2-3 times a day) during the growing season. Always error on the side of over watering... water fully til the water flows out the bottom of the pot, wait a few minutes and do it again.
I say it all the time, over and over, and over... Hoping a few people will get the point and avoid the losses I had because of this.
Recently, I lost a few young trees this Spring because I did not (Hell, COULD not) move everything in spduring a freak cold snap last month... a few Maples that had leafed out already got zapped and didn't come back... Most were fine and came back OK. I don't ever remember this type of snap freeze/SNOW so late in the year here!! Hope we never see it again!