Damage in skill score does not trigger a concentration check. Non-lethal damage does it.
By carefully observing the relevant rules, it appears that the damage to the skill does not actually harm the characterrather, it damages your scores.
Diseases, poisons, spells and other abilities can directly damage your skill scores.
Since this damage is affecting the scores of a character, and not the character itself, the damage does not count in the Injury section of the Concentration rules.
If you take damage while trying to cast a spell, you must do concentration control.
Non-lethal damage, however, is still damage to a character.
When you receive non-lethal damage, keep a cumulative total of how much you have accumulated.
Since damage is applied each time a character takes damage, receiving non-lethal damage requires a concentration check.