Is that to get people to pay quickly in order to minimize fluctuations in the exchange rate?
Yep. Bitpay guarantees a dollar conversion rate for 15 minutes. When a purchase is initialized a price is set in BTC that corresponds to a particular dollar amount. That way if the price of BTC goes down in the 10 minutes that it takes the customer to complete the purchase the seller won't lose money in BTC to USD conversion. Extending beyond 15 minutes opens Bitpay up to more risks.
Do you know if 15 mins is a static window or it can change or be changed in certain hours where volatility is higher?
Also is 15 mins based on some stats?