To prevent it from happening in the future, you have to burn the candle until the entire top of the wax has melted. That way when it dries, it’s all drying at the same level. I think the rule of thumb is an hour for each inch of diameter the candle is. It’s much easier to burn candles evenly that are on the smaller side obviously, since you can’t always let a candle burn for multiple hours. It sounds like the candle might be too big to avoid tunneling perhaps.
Side note- cutting your wick before each lighting and using a snuff to extinguish also help with keeping the wax clean in your candle.
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