There is no Scripture which I can find which sheds any light on this. However, there are several commentaries which do give an explanation. However, we have no way of knowing where they found their information. I have quoted them here for your consideration:
Whose blood Pilate had mingled ... — That is, while they were sacrificing at Jerusalem, Pilate came suddenly upon them and killed them, and “their” blood was mingled with the blood of the animals that they were slaying for sacrifice.
Whose blood Pilate had mingled — This piece of history is not recorded (as far as I can find) by Josephus: however, he states that the Galileans were the most seditious people in the land: they belonged properly to Herod’s jurisdiction; but, as they kept the great feasts at Jerusalem, they probably, by their tumultuous behavior at some one of them, gave Pilate, who was a mortal enemy to Herod, a pretext to fall upon and slay many of them; and thus, perhaps, sacrifice the people to the resentment he had against the prince. Archelaus is represented by Josephus as sending his soldiers into the temple, and slaying 3000 men while they were employed in offering sacrifices.
The Galileans whose blood Pilate had mingled with their sacrifices: We don’t have a record in secular history about the specific incident mentioned here. But there is a similar incident before the ministry of Jesus, Pilate wanted to build an aqueduct from the Pools of Solomon to the city of Jerusalem. To pay for it, he demanded money from the temple treasury, money that had been dedicated to God — and this outraged the people. When the Jews sent a delegation to beg for their money back, Pilate sent into the crowd soldiers dressed as common people, and at a certain signal they took out daggers and attacked the people asking for the money.
This doesn’t seem to be the same incident mentioned here, but it shows how completely consistent it was with the character of Pilate to slaughter some Galilean Jews on their way to sacrifice to the Lord in Jerusalem.