Sometimes playing under hot lights causes to stretch slightly and go flat. Tightening them up brings them back into tune.
Conversely, playing a cold venue which I have done on more than one occasion, causes the strings to shrink/tighten and go sharp. Hence the need to tweak again.
Also with some performers it's a nervous thing. The guitar isn't out of tune, but the singer needs to do something with his/her hands while they talk to the audience.

Seamus who still likes ol' Joanie