Cemetery SC gives dps buff for short period after capping both BOs. Clearly Glomburst dont crit so high normally, WOP might, but like u said, with 4k dmg cap, most likely it was sc buff.
Certain scenarios have mechanics that give temporary boost to DPS output, like Temple of Isha Warball or Highpass Cemetery lock bonus, which is somehow enough to "break" the 4000 dmg cap temporarily.
Most likely working as designed, as 4k hits rarely happen and the SC bonuses are also quite short lasting.