The root cause of the CWE-787 (Out-of-bounds Write) vulnerability in this code slice is:

1. Unbounded array access: The code increments the 'nrows' variable without checking if it exceeds the allocated size of the 'rows' array.

2. Lack of bounds checking: There is no validation to ensure that 'nrows' remains within the valid range of indices for the 'rows' array before performing the write operation.

3. Potential for buffer overflow: If the loop continues executing and 'nrows' grows beyond the array's bounds, it will write data to memory locations outside the intended buffer, potentially overwriting other data or causing undefined behavior.

4. Missing array size information: The code does not show the declaration or size of the 'rows' array, making it impossible to implement proper bounds checking within the given context.

5. Iterative error accumulation: The error accumulation logic in the loop may lead to more writes than expected, exacerbating the potential for out-of-bounds access.

To fix this vulnerability, proper bounds checking should be implemented before the array write operation, ensuring that 'nrows' never exceeds the allocated size of the 'rows' array.