I had this problem I changed the button and now no longer have this problem. Any other fix is a temperary fix. Unfortunately it's a difficult fix if you don't have the right equipment. For those that do add additional standard solder to the solder joints and then use two soldering irons to heat up all the switch solder points at the same time and remove the faulty button. Add more standard solder to the switch pads to lower the melting point and then remove with solder wick. Use amicro scope and fine tip soldering iron to solder the new switch in place. I would avoid using hot air as its very easy to over heat and damage these switches.

Replace the PCB board that contains the Micro-USB port (the one at the bottom). Search for the following. LG Nexus 5 USB port flex cable complete assembly. Make sure you get a complete assemble (which includes the black plastic) otherwise its a tough job transferring the double sided tape.