Could be a number of reasons:
She could have been using alternative tunings, but many guitarists doing this would bring other guitars with them, already tuned to the required notes.
Weather and heat of the lights on stage can affect your tuning quite badly and rapidly.
Nylon strings go out of tune quicker than metal strings.
New strings could still be stretching, and therefore go out of tune.
But she may also be one of a number of tuning-tweakers and twangers who seem compelled to do it at every juncture (viz. Dick Gaughan, Martin Carthy to name a couple) There are also those who make a big show of tuning and still end up out of tune! (not including the above-named in those necessarily!)
Hope you now feel in tune with this, Rara - and lucky you to see JB live!!