The recent eruption of Iceland’s Eyjafjallajökull volcano is not unique in displaying volcanic lightning that both originates and ends in the spewed volcanic ash — but the exact process isn’t quite nailed down yet:
If particles in a cloud exchange charge but remain close to each other, the cloud as a whole is still neutral. So there must be some process that separates the particles with positive charge from those with negative charge. This can happen if the particles have different aerodynamic properties.
For example, if positive charge tended to be concentrated in larger particles, those might fall faster than smaller particles. Gradually the positively and negatively charged particles would separate, with the larger particles lower in the cloud or toward the outsides of large eddies. This could create the charge separation that makes lightning possible.
The specific mechanism by which particles of differing charge are separated is unknown. This idea of particle size segregation is just one possibility. It seems likely that other processes are involved.