There are two different references in this passage from Luke’s Gospel, one a historical episode involving the suppression of a riot; the other an allegory to illustrate repentance.
In both the message is: change your lives, because the Endtime is coming, and even more suddenly than you think.
First, the historical event: it seems that the Roman governor Pilate suppressed a riot in the Temple involving Galilean zealots with great cruelty; “he mixed their blood with that of their sacrifices”, as Jesus puts it. Jesus uses this example to stress that the victims of Pilate were not necessarily worse than anyone else. It’s not that they deserved their fate.
Around the same time, some eighteen people died when part of a tower in Jerusalem collapsed on them. Nor were these people worse than others, says Jesus. Their death was not a punishment.
The point is, Jesus says, the Endtime draws near, and anyone and everyone can be caught up in death and disaster. So be ready, he tells his disciples. Repent! Change your ways! Accept God’s will for your life. Don’t think, there’s plenty of time left. No, there isn’t, because you have been given time, but have not put it to use.
The allusion passes on to the fig tree which bears no fruit, and which the master wants to uproot. The gardener begs for a little extra time, and his wish is granted. The tree will stay for a year longer, no more.
These passages in Luke are ‘eschatalogical’. They refer to what happens at the end of time – eschaton means ‘end’. Sudden or gradual, we can’t escape our end. The proper attitude therefore is – be prepared! Live righteously!