Casting and dereferencing of pointer

Maybe I should not post in this group but anyway...

Please look at my code that do the same, except of endianness.
static void _parseItems(const unsigned char *pBuffer)
{
  unsigned int itemSize;
  itemSize = *((unsigned int*)pBuffer); // this give system fault
  memcpy(&itemSize, pBuffer, sizeof(unsigned int)); // this works well
  .......
}

I'm not very experienced with C so I use git as example of good written code. In object.c I've found two functions that looks like my one.

static unsigned int hash_obj(struct object *obj, unsigned int n)
{
	unsigned int hash = *(unsigned int *)obj->sha1;
	return hash % n;
}

static int hashtable_index(const unsigned char *sha1)
{
	unsigned int i;
	memcpy(&i, sha1, sizeof(unsigned int));
	return (int)(i % obj_hash_size);
}

I wonder why in the second used memcpy instead of: unsigned int i = *(unsigned int *)sha1 Maybe there is explanation that will help to solve my problem. Thank you