Heya,
Apex Coin balances can go in the negative due to various of reasons, one of the more common ones if is a recent coin purchase has been refunded or if a chargeback has been issued. (After the coins have been spent)
If you get a refund in real-world currency, we’ll take an equivalent amount of Apex Coins from your Wallet. If you don’t have enough Apex Coins to cover your refund, your balance will be negative.
New purchases of Apex Coins will first be applied to your negative balance.
From: [EA Help] - How to buy, use, and find Apex Coins and Exotic Shards