Management (Bezos) apparently refused to hire an adequate number of pilots (unable?) to accommodate the dramatic increase in flying hours from the Amazon contract. Possibly out of fear Amazon (20 B767s) would pull out, timeline was too short to staff, or because he's trying to maximize profitability by running an overly lean and understaffed operation.

Either way, Amazon is learning a lesson: air ops require pilots that can't be hired and fired for seasonal expansion/contraction. In addition to ensuring its own parcel delivery service was secure and prioritized, Amazon was likely trying to send a message to UPS/FedEx that they could source outside help if needed. Warning shot backfired. Welcome to the big leagues.