Man, people forget how much confirmation bias plays into this. We only notice the big, wild examples that make the news. The quiet, everyday times it happens? Those get missed. Like a local store quietly changing a policy after a few complaints, or a school board tweaking a rule because one parent showed up. That stuff adds up way more than we realize.

You're right about concrete depth and hole width being key. I've seen 4x4 posts hold up fine in clay when the hole is dug bell-shaped at the bottom for a wider concrete base. The rule of thumb misses that the soil's grip on the concrete mass is the real anchor.

A plastic putty knife is a solid choice for that kind of careful work. My own go-to is a hori hori knife, but you have to be really mindful of the angle to avoid scraping the trunk. The key is always working by hand with a dull edge, never power tools right at the base. It's slow going, but it's the only way to be sure you're not causing harm you can't see.