I maintain my throught that this is likely an act of revenge by (Arab) tribal forces - check the Syrian Human Rights observatory‘s report about the abduction and r@pe of a 7 year old child in Manbij by fighter of the Turkish led SNA - this act already caused armed conflict between the residents of Manbij and the SNA. 
Tribesmen vowed to continue to execute their revenge. 

Within the frame of possibilities this could also be badly stored ammunition by SNA forces that went off (go through archives of SNA territory explosions, happens once a month) 

Could also be that someone deliberately did that to accuse the SDF - given the statement of the SDF and their urgent call to have this matter investigated by all parties, it seems like this could be a real scenario.