Let's analyze the code step by step to identify the CWE-787 (Out-of-bounds Write) vulnerability:

1. Input variables:
   - k, n: uintmax_t (unsigned integers)
   - buf: char pointer (destination buffer)
   - bufsize: size_t (size of buf)
   - initial_read: size_t
   - file_size: off_t

2. Calculated values:
   - start = (k - 1) * (file_size / n)

3. Condition check:
   - if (initial_read != SIZE_MAX || start < initial_read)

4. Vulnerable operation:
   - memmove(buf, buf + start, initial_read - start)

5. Potential issues:
   - The condition doesn't ensure that 'start' is less than 'initial_read'
   - There's no check to ensure (initial_read - start) is non-negative
   - There's no bounds checking for 'buf' or 'buf + start'

6. Vulnerability analysis:
   - If 'start' > 'initial_read', (initial_read - start) becomes negative
   - A negative value, when cast to size_t (unsigned), becomes a very large positive number
   - This large number is used as the size parameter in memmove()
   - memmove() will attempt to copy this large amount of data, leading to a buffer overflow

7. Out-of-bounds Write:
   - The large size parameter causes memmove() to write beyond the bounds of 'buf'
   - This results in overwriting adjacent memory, potentially corrupting data or code

Conclusion:
The vulnerability occurs because the code fails to properly validate the relationship between 'start' and 'initial_read' before using their difference as a size parameter in memmove(). This can lead to an out-of-bounds write, potentially overwriting critical data or enabling arbitrary code execution.