Because he (Shiva) was very much desirous of seeing her, therefore when she came at his side, another face came out on the southern side of his body with eyes like fullblossomed lotuses.
द्रष्टुकामस्य चात्यर्थं गतया पार्श्वतस्तया। अन्यदञ्चितपद्माक्षं दक्षिणं निःसृतं मुखम्॥