Using the search tool at the Unbound Bible site, or any similar site/application, it can be seen that μαρτύρων is rendered in the KJV as "witnesses" or "testifies" in all but one instance, Revelation 17:6, where it is given as "martyrs".
In regard to Revelation, my guess is the KJV translators chose "martyrs" because their blood has been shed for their testimony. It could be, though, that these particular "witnesses" are the "prophets", since the writer of Revelation refers to "saints" and "prophets" in Revelation 11:18, 16:6 and 18:24 -- "saints and martyrs" or "saints and prophets", either way works, since both have had their blood shed for their testimony.
The saints (αγιων - holy ones) have died for their testimony, too, but they have distinguished themselves in other ways, such as their prayers (Revelation 5:8,8:3-4), patience and faith (Revelation 13:10, 14:12), and their righteousness (Revelation 19:8).
There are two occurrences of the singular form of the noun, μαρτυρος. One in Acts 22:20 in reference to Stephen, and the other in reference to Antipas in Revelation 2:13.
The circumstances recorded in Acts surrounding the death of Stephen, possibly explains the preference for, and hence the growth in, the usage of the word "martyr".
The pursuit of martyrdom is something that should be discouraged. Stephen was killed for the "witness" of his faith in Jesus. He is referred to as a martyr, but making martyrdom his pursuit would have been the furthest thing from his mind.
Here's the issue with the notion of martyrdom:
So it befell the blessed Polycarp, who having with those from Philadelphia suffered martyrdom in Smyrna--twelve in all--is especially remembered more than the others by all men, so that he is talked of even by the heathen in every place: for he showed himself not only a notable teacher, but also a distinguished martyr, whose martyrdom all desire to imitate, seeing that it was after the pattern of the Gospel of Christ.
-- The letter of the Smyrnaeans 19:1
How many times does "martyr" appear in this passage in comparison to "Christ"? It's 3 to 1, and "Christ" is only a peripheral reference.
Here is a danger to be avoided: martyrs are glorified, and if they are glorified, Christ is not. In the letter from which the passage comes, there are five "Blessed Polycarp"s, three "holy Polycarp"s, a "glorious Polycarp" and a "glorious martyr Polycarp".
μαρτύρων refers to "witnesses", i.e. those who testify in word and deed to the goodness of God, regardless of the consequences. In the one place in Revelation where the KJV has given "martyrs", there is sufficient evidence for them to have chosen "prophets" instead, with a footnote indicating the word is actually "witnesses".