The constant rebooting problem is not isolated to the Nexus 5 but also the Samsung Galaxy S3 & S4 and other models which use the same part.
The offending part
The switch works on the principle of pulling the circuit low (to ground), booting the phone or bringing it out of standby.
Underneath the plastic pusher is a soft malleable nipple to depress the buckles
Bottom BuckleI do not understand why there is a solder blob on the switch buckle, other than to help make contact with the buckle and the switch PCB.
While the solder blob isn't the switches direct cause of failure, the fact the blob is off center is!
Solder is soft, over time and continual depressions it can flatten out. Not to mention solder leaves residue which can build up over time.
Below you can clearly see how the solder has transferred to the PCB, even where there is no copper trace, resulting in completing the circuit to ground, Athlon XP pencil trick anyone ?
While the original LG part, manufacturer and origins are unknown, the problems associated with reboots are almost certainly a manufacturing fault of the switch.
It must be noted, the repair was very fiddly! and took two replacement switches before the phone was fixed. The first failing within a day, the second switch remains working to this day.
Failure is always an option : Adam Savage