What does it mean when there are rewards with negative values in the block's balance_updates?

Question

When querying a block via RPC, the balance_updates array found inside the metadata object may have entries with negative rewards. For instance:

{
   "kind":"freezer",
   "category":"rewards",
   "delegate":"tz2KuCcKSyMzs8wRJXzjqoHgojPkSUem8ZBS",
   "level":29,
   "change":"-542000000"
}

So, what does it mean when a delegate get a negative reward?