The CWE-787 vulnerability is an "Out-of-bounds Write" vulnerability. This occurs when the code writes data to a memory location that is outside the boundaries of a buffer or array.

In this specific case, the vulnerability is caused by the line `rows[nrows++] = br + bpr*sy;`. 

The root cause of this vulnerability is likely due to the fact that the `rows` array is not being bounds-checked before writing to it. The `nrows` variable is being incremented without checking if it exceeds the maximum allowed index of the `rows` array.

This can lead to a buffer overflow, where data is written to memory locations outside the `rows` array, potentially causing undefined behavior, crashes, or even code execution.

To fix this vulnerability, the code should add a bounds check before writing to the `rows` array, for example:

```c
if (nrows < MAX_ROWS) {
    rows[nrows++] = br + bpr*sy;
} else {
    // Handle error or overflow condition
}
```

Where `MAX_ROWS` is the maximum allowed size of the `rows` array.