Why do Catholics pause at the end of the Lord’s Prayer for the priest to add his part and then finish?

What the priest says at the end of the Lord’s Prayer is not an interruption but a prayer continuing the final sentiment of the Our Father. The doxology, which follows that second prayer – “For the kingdom, the power and the glory are yours now and forever” – was never part of the Lord’s Prayer. Through a mis-translation of the Missal after the Reformation in England, the “Deliver us O Lord...” prayer was omitted, and the doxology was put adjacent to the Lord’s Prayer. Over the course of time, it morphed into being part of the prayer, and is the common usage in many of the Protestant and Evangelical churches.