Maybe they like to hang out there.
Seems like "under the altar" would have significance.
The phrase is used in the sacrificial system thusly:
And the sons of Aaron brought the blood unto him: and he dipped his finger in the blood, and put [it] upon the horns of the altar, and poured out the blood at the bottom of the altar: [Lev 9:9 KJV]
And thou shalt offer thy burnt offerings, the flesh and the blood, upon the altar of the LORD thy God: and the blood of thy sacrifices shall be poured out upon the altar of the LORD thy God, and thou shalt eat the flesh. [Deu 12:27 KJV]
And here are two verses referring to Christ being poured out, as a sacrifice for us:
I am poured out like water, and all my bones are out of joint: my heart is like wax; it is melted in the midst of my bowels. [Psa 22:14 KJV]
Therefore will I divide him [a portion] with the great, and he shall divide the spoil with the strong; because he hath poured out his soul unto death: and he was numbered with the transgressors; and he bare the sin of many, and made intercession for the transgressors. [Isa 53:12 KJV]
So it would appear that the souls "under the alter" were "poured out" like Christ was, suffering like He did, as Christ expected of His disciples:
Remember the word that I said unto you, The servant is not greater than his lord. If they have persecuted me, they will also persecute you; if they have kept my saying, they will keep yours also. [Jhn 15:20 KJV]