What could be thread unsafe about this?

thread.cs

 // When this function runs in my program on multiple threads, memory corruption eventually happens
 // When it doesn't run, or runs on only 1 thread, no problems
 // I can't see what could be thread unsafe about it, am I missing something?
 public static float SingleCellular2EdgeSimple(float x, float y, float jitter, int seed)
    {
        int xr = (x >= 0) ? (int)(x + 0.5f) : (int)(x - 0.5f);
        int yr = (y >= 0) ? (int)(y + 0.5f) : (int)(y - 0.5f);

        float[] distance = { 999999f, 999999f, 999999f, 999999f };


        for (int xi = xr - 1; xi <= xr + 1; xi++)
        {
            for (int yi = yr - 1; yi <= yr + 1; yi++)
            {                
    
                int hash = seed;
                hash ^= 1619 * xi;
                hash ^= 31337 * yi;

                hash = hash * hash * hash * 60493;
                hash = (hash >> 13) ^ hash;

                //Float2 vec = new Float2(0.5f, 0.5f); // CELL_2D[hash & 255];

                float vecX = xi - x + 0.5f * jitter;
                float vecY = yi - y + 0.5f * jitter;

                float newDistance = vecX * vecX + vecY * vecY;

                for (int i = 1; i > 0; i--)
                    distance[i] = Math.Max(Math.Min(distance[i], newDistance), distance[i - 1]);
                distance[0] = Math.Min(distance[0], newDistance);
            }
        }


        return distance[1] + distance[0];


    }