In order to make the nose go down (blades low at the 12 o'clock) the swashplate must be tilted down towards the 3 o'clock position to allow for phase lag. That sudden acceleration due to forward cyclic flapping the blades down on the right hand side of the disc might be a factor in this right roll - I don't know, but it may be worthy of consideration - acceleration cross-couples are not unusual in helicopters.