No, its not just Christmas, I had this happen the last time too!.UPS is just soooo busy now with Christmas packages they are recruiting help from USPS. You may also notice they are using Penske trucks also in certain locations but just during this time of year.
UPS hands off small packages to USPS based on size, weight and delivery efficiency. Some of their sorting facilities are literally combined and is an amazingly efficient operation. This is a year around operation and not just during peak times. I have been involved in building these centers and it is really impressive to watch the operation- detailed, smooth and fast