Andrew Gabriel wrote:

Sounds like a tree which had gone rotten, and should have been
spotted and removed as part of regular park maintenance, which
very likely isn't happening as much as it used to.

Having had a number of shed limbs in the car park at work and trouble
overcomming our estates dept's inertia I went direct to Southampton City
Council and their tree people addmitted that most of the cities trees
hadn't been looked at for almost 30 years.
School and major road trees are now being looked at first, we were told
we might be lucky and they might look at the trees in 3-4 years, then a
big bit fell, with the threat of the HSE getting involved they had a
crew out the same day to take the rest of it down, the others however
will still have to wait those 3-4 years....