(from e-Sword, Clarke's commentary)
This shows that the prophet did not perform his miracles by any powers of his own, but by the power of God; and He chose to honor his servant, by making even his bones the instrument of another miracle after his death.

(from ESV study bible):
2 Kings 13:20–21 grave of Elisha. Tombs in ancient Israel were often dug out of soft rock, or located in caves (e.g., Genesis 23), and they were not difficult to access. It is probably important to know at this point that Elisha's powers to resurrect live on (cf. 2 Kings 4:8–37), because as this man was thrown (Hb. shalak) into the grave of Elisha, so God will soon “throw” (or “cast”) Israel into exile in Assyria (17:20, same verb, shalak). The Israelites need to maintain contact with the great prophets of the past through obedience to their teachings if this “death” in exile is also to be followed by an unexpected resurrection (cf. Ezek. 37:1–14).

from EBC(Expositor's Biblce Commentary)
20b-21 One last miracle would attend God's faithful prophet. In those last dark days before stability was restored to the area by Adad-Nirari III, bands of Moabite marauders would ravish the land at the beginning of the harvest season (v. 20b). Evidently Elisha had died at such a time. On one occasion, as a funeral procession made its way to the burial place, a looting party swooped down on them (v. 21). In their need for swift flight, the members of the procession quickly halted and hastily placed the body of the dead man in the first available tomb--which happened to be Elisha's. When the burial party lowered the linen-wrapped body of the dead man into the tomb, it came into contact with the remains of Elisha. Instantly the man was revived. The juxtaposition of this event with the account that precedes makes it clear that herein was another divinely intended sign for Jehoash and Israel: God was the God of the living, not the dead (cf. Luke 20:38), not only for Elisha and the man who had been restored to life, but for Israel as well. Israel could yet "live" if she would but appropriate the eternally living God as her own. The entire episode was, further, a corroborative sign that what Elisha had prophesied would certainly come to pass. Only a living God could guarantee such a thing (cf. Isa 44).