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One common occurrence I have found several times in this situation is when a drain line (usually a storm water drain of some sort) runs under the concrete and has a hole or a bad joint in it allowing water to exit and re-enter the hole taking some of the dirt with it. This will cause a void under the concrete over time and until it is repaired will continue to worsen. This is only one of many possibilities though. If there is a downspout drain or any other type of drain that you know of in the immediate vicinity it would be a very distinct possibility.