Is this mention of "people from another world" a reference to something in Pratchett's "Thud!"?

Question

In Terry Pratchett's Thud!, when Vimes and company are heading to Koom Valley with the assistance of the Unseen University, there's this passage with exploding vegetables:

'Don't try!' Vimes said quickly. 'And I swear I saw a cow explode back there! Keep us away from towns and people, will you?'
Behind the coach, turnips and rocks leapt into the air and bounced away in the opposite direction. Vimes hoped they wouldn't get into trouble about that.*
The other thing he noticed was that the landscape ahead was strangely bluish, while behind them it had a relatively red tint. He didn't like to point this out, though, in case it sounded strange.

*_{But as it happened, it was all blamed on people from another world, so that was all right.}

The bit about red and blue is obviously a reference to redshift and blueshift. The footnote, however, I don't quite understand. Generally, when Pratchett has a footnote like this, it's a reference to another work of literature or something in that vein; is that the case here? If so, what's it a reference to, and if not, what's this talking about?

Answer 1

Alt.fan.pratchett gives this annotation for the footnote:

For some reason, UFOs are sometimes blamed for mysterious mutilations of cattle,

so it seems that it's not the "turnips and rocks" Vimes was worried about, but the exploding cow. But exploding cows typically get attributed to UFOs.