it's likely that the person used a mesmer skill that mimicked one of their allies skills or removed an enchant from an enemy... (like revealed enchantment)
The Necro in your pic has Mesmer as a secondary. Check up on the following Mesmer skills:
Arcane Larceny - Spell
For 5-29 seconds, one random Spell is disabled for target foe and Arcane Larceny is replaced by that Spell.
Arcane Thievery - Spell
For 5-29 seconds, one random spell is disabled for target foe, and Arcane Thievery is replaced by that spell.
Inspired Enchantment - Spell
Remove an Enchantment from target foe and gain 3-13 Energy. For 20 seconds, Inspired Enchantment is replaced with the Enchantment removed from target foe.
Revealed Enchantment - Spell
Remove an Enchantment from target foe and gain 3-13 Energy. For 20 seconds, Revealed Enchantment is replaced with the Enchantment removed from target foe.
These spells can be pretty nifty if you manage to snatch one from your own attribute lineup. In addition, you will be disabling a skill from an opponent. I goofed around with Arcane Echo and Arcane Theivery in Fort Aspenwood recently when I needed the 10,000 Luxon faction for befriending the Luxons quest. At one point I "borrowed" two elite skills from enemy necros and had three different elites on my bar.