This seems to align well with the general halakhic principle of ḥumrot which prefers a more restrictive application of Torah commandments to avoid their potential violation (the “fence around the Torah” principle). An early example of this is the rendering of Leviticus 24:16 in the LXX which motivates the restriction on pronouncing the divine name Yhwh; another example is the avoidance of meat with dairy in dietary kashrut on the basis of Exodus 34:26 (Hullin 115b). It is unclear whether crucifixion was introduced into Judea under the Achaemenids (who used suspension in execution but not in the same way as the later Romans) or the Hasmoneans but certainly by the first century BCE the commandment in Deuteronomy 21:22-23 was interpreted as applying to it. The passage in 4QpNahum is interesting because although the source text refers to postmortem suspension, the allusion in the pesher utilizes the language from Deuteronomy 21:22-23 but inserts the word חי to clarify that it concerns those who are suspended alive on trees (see the discussion in Max Wilcox’s 1977 JBL article, p. 88). This is a more restrictive reading that applies the passage beyond the original implied situation to cover other cases where the law may also apply. Since non-compliance would result in a defiling of the land (תטמא את אדמתך) from the curse borne by the corpse, it is not surprising that a more stringent application was preferred.
Suspension was certainly part of the word’s meaning, as its usage in Job 26:7, Psalm 137:2, Song of Solomon 4:4, Isaiah 22:24, Ezekiel 15:3 shows. Militating against an impalement interpretation is 2 Samuel 18:10 which suggests hanging on a living tree (a terebinth or oak), as opposed to penetration by a sharpened stake. On the strength of rabbinical parallels, Joseph Baumgarten (JBL, 1972) suggested that the Qumran usage referred to strangulation (hanging by rope), and not crucifixion. But this is unlikely because 4QpNahum was referring to a historical event concerning Alexander Jannaeus also reported by Josephus who characterized the punishment as crucifixion (AJ 13.380). You can find a full discussion on the meaning of תלה in this context in John Granger Cook’s Crucifixion in the Mediterranean World (pp. 315-326; Mohr Siebeck, 2014). He also points out that the rendering of Deuteronomy 21:22-23 in the LXX (with κρεμάννυμι) is better understood as referring to suspension or crucifixion rather than impalement (pp. 226).