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

The function signature uses `intptr_t` for the `load_addr` parameter, while using `uint32_t` for `data_addr` and `data_len`. This mismatch in data types can lead to potential buffer overflow issues, especially on 64-bit systems where `intptr_t` is 64 bits wide, but `uint32_t` is only 32 bits.

Explanation:
1. On a 64-bit system, `intptr_t` is 64 bits wide, allowing for a much larger address space than `uint32_t`.
2. If `load_addr` points to a memory location beyond the 32-bit address space, but `data_addr` and `data_len` are limited to 32-bit values, it could result in accessing or writing data beyond the intended buffer boundaries.
3. This mismatch can cause the function to write data to unintended memory locations, potentially overwriting other data or causing memory corruption.

To mitigate this vulnerability, the function should use consistent data types for all address and size parameters, preferably using `size_t` for sizes and appropriate pointer types for addresses. Additionally, proper bounds checking should be implemented to ensure that all memory accesses remain within allocated buffers.