When a tree falls on Highway 29, usually there are plenty of people around to see or hear it.

That was the case Wednesday, when one in a grand stand of eucalyptus trees north of Napa toppled over the highway, stopping southbound traffic and presenting an unusual task for work crews. We are lucky that such a large object crashing onto such a busy road hurt no one.

What causes trees to fall?

Less than two weeks ago arborist Bill Pramuk, Register “Trees and People” columnist, submitted a timely piece headlined “Summer tree failures.”

He pondered what causes limbs to suddenly crash, and mentioned a few likely scenarios. One is “sudden limb drop,” also known as “high temperature drop.” While the evidence of a problem is straightforward — falling limbs on hot days — Pramuk said arborists do not know a cause for this phenomenon.

It is worth noting that the high temperature in Napa Tuesday was 97, though temperatures cooled a bit Wednesday.

Another possible culprit, Pramuk said, is a high wind, though the breezes were not strong Wednesday.

Meanwhile, there has been trouble in this mighty grove of eucalyptus before.

The most notable tree in the stand between southbound and northbound traffic has been subject to medical treatment in recent years. Four years ago another arborist, Joe Borden, began treating the tree known as the “pregnant lady” or the “naked lady” — the northernmost in the stand and one with a massive bulge at the lower end of the trunk — for an infestation of a bug known as lerp psyllid.

The treatment is apparently helping, as that old tree still stands.

But, to borrow from Pramuk’s column: “With the onset of hot weather and the unusually heavy growth of foliage in many trees around the valley this year, it is critical for tree owners and property managers to take a close look at trees under your stewardship, consider the level of risk you are willing to live with and take action. If it looks suspicious to you, it probably warrants a professional opinion.”