Ligthning Mastery/GCD broken?

[tryll]

I've been experiencing (in BGs) that some casters seem emit absurd amounts of spells in short time. Since I don't know every talent/ability/mystic enchant (combination) I can usually only dig through the combat log for understanding.

I've managed to capture the following log:

Here red has 5 spells (white arrows) hit me within 2 seconds (white box) of the combat log. Casting of the first spell could have started beforehand, which still means 4 spells cast within 2 seconds. The truly strange spells are the 2nd and 3rd Lightning Bolt hits (white arrow 2 and 4) - without them this would look quite normal.
Now I'm wondering how this could be possible. The options I see:

1. Lighting Mastery (epic mystic enchant) is broken: The secondary Lightning Bolt hits (white arrow 2 and 4) are it triggering. Then the broken part is that it is supposed to only deal half damage which evidently is would not be the case (unlike Lightning Overload, whose damage is correctly halved). All this is under the assumption that Lightning Mastery does not have a dedicated combat log entry unlike Lightning Overload, which I cannot confirm.
2. The GCD is broken/not capped at 1 second.
3. Some other talent/ability/mystic enchant does enable (instant?) cast(s) of Lightning Bolt, possibly without GCD.

Does anybody have similar experiences/observations or an idea what is going on in this particular case?

PS: I'm a bit baffled that one can output 9.5k damage in 2 seconds - that seems a tad excessive. For context, I have 5/5 (set) + 3/3 (weapons) + 1 (off) pieces of Merciless Gladiator gear which puts me at 134 resilience.