Salzella shrugged. 'We've got to do this properly. Did you know Dr Undershaft was strangled before he was hung?'
'Hanged,' said Bucket, without thinking. 'Men are hanged. It's dead meat that's hung.'
'Indeed?' said Salzella. 'I appreciate the information. Well, poor old Undershaft was strangled, apparently. And then he was hung.'
Bucket isn't the only one who makes a distinction between hung and hanged: hanged for people who are killed by hanging, and hung for everything else. No reason is given for this prescription, and it is not usually followed anyway. Merriam-Webster's Dictionary of English Usage provides many examples of the words being used interchangeably, and concludes that if you observe the distinction, "you will spare yourself the annoyance of being corrected for having done something that is not wrong."
hang has two forms of the past tense/past participle: hanged and hung. This is because it was originally two verbs:
the Old English strong verb hōn, past tense heng, past particle hangen "to hang"
and the Old English weak verb hangian, past tense hangode "to be suspended" - this is where the hang and hanged forms come from.
It seems that these verbs were originally a transitive-intransitive pair, like lay - lie and sit - set.
hōn and hangian are from the Proto-Indo-European root *ḱonk- "to hang", which is also the source of hanker. In Latin it's cūnctārī "to delay", as in cunctation which means "the action of delaying; delay, tardy action."
By the 14th century, the two verbs had collapsed into one: hangen, past tense heng, hong, hanged, past participle hanged. By the late 16th century, the past tense forms had become hung (by analogy with sing, sang sung) and hanged. However, the OED notes that Northern England dialects still have two verbs: hing, hang, hung and hang, hang'd, hang'd - the second one is reserved for referring to death by hanging. German also preserves a formal remnant of the older transitive-intransitive distinction with the verbs hangen and hängen, which both mean "to hang" and which are used interchangeably.