The ancient Greeks were famously suspicious of foreigners. This suspicion runs all through The Bacchae. The play explores the dangerous tensions that arise when a foreign religion starts to take purchase in a new land. When the cult of Dionysus meets resistance upon arrival in Greece, no end of trouble happens. What complicates the issue in the play is that the god Dionysus is Greek himself. In a way he's foreigner in his own home. The play is full of these sorts of interesting dualities.
Pentheus exhibits typical Greek prejudice towards foreigners.
The spread of the cult of Dionysus is a foreign invasion.