Let's do it with maths:
Interrupt lasts for 2-3 seconds, casting takes maximum 3 (3.0016 for a dex-debuffed, unsc'ed, unbuffed level 20 half ogre). This means that the slowest attack you can keep someone constantly interrupted with is less than 6s.
Pet hits every 3 seconds (with a one-hand weapon), so it can interrupt one caster. It can't interrupt two, because by the time it's made it back to the first, it'll have taken 6s - long enough to get a cast off.
Lifetap every 4 seconds to interrupt one caster. It can't interrupt two, because by the time it's made it back to the first, it'll have taken 8s - long enough to get a cast (or two) off.
Melee every 4 seconds to interrupt one caster. It can't interrupt two, because by the time it's made it back to the first - it'll have taken 8s, long enough to get a cast (or two) off.
I can believe that you killed 6 casters, but they played incredibly bad and you can't have interrupted them all.
Edit: I know that I assume they cast synchronous, so you might be able to interrupt all 6 for the first round, but after that the above calculations hold true.
Interrupt lasts for 2-3 seconds, casting takes maximum 3 (3.0016 for a dex-debuffed, unsc'ed, unbuffed level 20 half ogre). This means that the slowest attack you can keep someone constantly interrupted with is less than 6s.
Pet hits every 3 seconds (with a one-hand weapon), so it can interrupt one caster. It can't interrupt two, because by the time it's made it back to the first, it'll have taken 6s - long enough to get a cast off.
Lifetap every 4 seconds to interrupt one caster. It can't interrupt two, because by the time it's made it back to the first, it'll have taken 8s - long enough to get a cast (or two) off.
Melee every 4 seconds to interrupt one caster. It can't interrupt two, because by the time it's made it back to the first - it'll have taken 8s, long enough to get a cast (or two) off.
I can believe that you killed 6 casters, but they played incredibly bad and you can't have interrupted them all.
Edit: I know that I assume they cast synchronous, so you might be able to interrupt all 6 for the first round, but after that the above calculations hold true.