It looks like turbulence or presure spikes on the chamber at ignition, depending the magnitude that could lead to a explosion (I just an aficionado, so take this opinion for what it´s worth) I also follow the sugar rocket to space project, and I think they had some problems with this, and with cracking in the propellant. Is the data from the launch available?.
Does anyone know why the flames appear to "stutter" from the rocket in slow motion? Is that expected behavior?