He might have been fired for the misconduct he lied about, although I think it’s likely he might have been disciplined but not fired. Lying in an effort to cover it up fits the usual mould that the cover up may be seen as worse than the offense.
Why is lying a firing offense in itself? Well, it is in police departments because cops get lied to on a regular basis. They know it and they know it comes with the job. But they don’t want to have to put up with it from one of their own.
This is easy to understand if you know a couple of things about human psychology. We reserve our most viseral dislike, hatred even, for those whom we are close to when we think they have betrayed us. Its much easier to take the betrayal of someone who you would expect to betray you. But when it is someone you trusted because of a special relationship you have with that person, it’s much different.
This is why divorces are nasty. It’s why cops get fired for lying to their comrades and bosses even when the underlying conduct would not have been a firing offense. The relationship among cops is different. They fight together and they might die together. Deception from someone in such a special position of trust is not easily tolerated. It’s like this: “I have his back, he’s supposed to have my back. He’s not supposed to lie to me.” Because if he does you might think you can’t trust him to have your back anymore in the dangerous world in which you both work.