While you never really know with e-Rewards, it seemed like last time negative balances happens, it was because a batch of BA awards never properly made into Avios accounts. A few weeks later, the failed redemption got credited back to e-Rewards accounts and then a week or two later, that batch of redemptions got pulled again from accounts (for successful redemption of Avios that time) whether there was enough of the balance for redemption or not, leading to sone folks with short term negative balances.