Well, for older, European opera houses (such as the Opéra de Lumiose), it’s not exactly a common occurrence, but it is one that isn’t unheard of. Places such as those tend to attract a lot of litwick and their evolutions, and some individuals may get more restless or mischievous the more people are there to surprise.
As with most ghost-types, tamed chandelure are often less likely to engage in this kind of behavior, as training will naturally help a ghost-type wean off this need to scare others. The reason why is because the need to scare is often linked to either a ghost-type’s hunting techniques (as startled prey is easier to disable and feed on) or a ghost-type’s need for enrichment. Steady diets and a reliable source of both entertainment and mental engagement typically satisfy a ghost-type’s needs, so you will rarely see haunting behavior from a trained ghost-type except in mild examples or in cases of neglect.
In short, it’s not surprising that a chandelure dropped in on the performance (forgive the pun). It’s more surprising that the Opéra de Lumiose has apparently let its litwick infestation go so far as to produce a mischievous chandelure. Normally, one would think an opera house would round up and tame its resident ghost-types as much as possible to avoid interruptions of performances.
But of course, that’s neither here nor there.