Mulching trees...
I have the mind-numbing job of going out to look at trees thanks to trouble-calls from people who think they have EAB, etc.. Over 50% of the trees I end up inspecting, are dying from home owner stupidity. Mulch volcanos, planting depth issues and weed whippers are prevalent.
Mulch volcano - people think that a little much is a good thing, so a huge pile must be a great thing. Don't pile mulch up the trunk of the tree!
Planting depth - When you plant a tree, you should see the flare at the base of the trunk. Don't bury it! Sure, the tree my grow for a few year but it will become distressed and then you'll have a half-grown tree that is really close to being the nice shade tree you wanted, annnnnd then it will croak.
Topping - At least once a year I will see a truck from a tree care company, that has a big advertisement on it about topping - "Call us for your topping needs!" And, each time I nearly black out.
'Lead poisoning', or at least that's what I call it. Don't weed whip around the trunk of your trees, or ram your lawnmower into them. I live by a park that has a bunch of half grown trees and they've all dying from the lawn guy whipping the living daylights out of the base of every one of them. It makes me sad.
And of course, I run into things like trees planted years ago but the wire basket around the roots was not removed (rood girdling), or severe lighting damage that the owner though would just 'heal over', trenching or some other activity that cut many roots, and of course, the deadly flower bed. Staking up bricks, filling with soil and planting flowers/hostas/whatever around the base of a tree. It's the same as planting a tree too deep.
If you don't know what you're doing, or the tree is very large, I highly recommend a certified arborist. Just because someone hangs out a shingle that proclaims they know what they're doing ... doesn't mean they really know what they're doing. Make sure they're bonded/insured.
And, personally, I never used rock mulch around plants, ever. Rocks hold a lot of heat and the plants literally fry. I invest too much $$ and time in my yard to intentionally kill them.