When a player leaves a guild, his/her total contribution is deducted from guild assets. So, if total contribution leaving the guild is higher than current guild assets, you would have a negative amount. Did your guild just level up and/or a few players left (removed) the guild?
Player Game Master Server: EU45 Level: 113 IGN: ♛Aizen♛ BP: 927,224 Reiatsu: 79