There are very good reasons that the FCC polices the use of wireless networks and spectrum as carefully as it does. Unauthorized operations can cause interference with other services both near and at the station being used. Because of this, operating any kind of wireless network equipment either without a license or outside of the terms of the license granted by the FCC is not only dangerous, it's against the law. Thus, when it came to light that AT&T seemed to be operating multiple fixed microwave stations outside of the terms of their licenses, the FCC took investigation into the matter pretty seriously. With findings so far pointing to no harm having been done, AT&T has made corrections to the requisite operations, and has laid $450,000 on the table in order for the FCC to close the books on the matter.
The investigation started back in 2012, and looked at a number of acquisitions and station uses that AT&T had made between 2009 and 2012. Many of these fell squarely outside the terms of the licenses that AT&T had been granted for a given region or piece of equipment. According to the FCC, AT&T failed to check over the licenses before beginning operations, resulting in unauthorized use by their subsidiaries, AT&T Mobility Puerto Rico and New Cingular Wireless PCS.
It is worth mentioning that the FCC's initial findings on the matter were not entirely accurate. AT&T managed to dig up a number of discrepancies between the FCC's claims, the licenses they were granted, and their operations during the time the investigation covers. They handed over this list of discrepancies, along with a few corrections to their earlier filings on the matter, and their proposal to have the whole matter simply go away in exchange for $450,000. While it's entirely possible that AT&T may have had a case to either bring their fines below that level or disprove the FCC's findings entirely, it is also quite possible that AT&T decided that the money and manpower it would take to fight the case made it a better option to simply lay a decent amount of money out and see the matter dismissed outright.