Wiki page of 牛排 gives a clue of its etymology, written by, 姚德懷, the current chairman of 香港中國語文學會 (The Chinese Language Society of Hong Kong Ltd.), a non-profit organization in Hong Kong. Here's a summary:
According to 漢語大詞典, the word 牛排 has been cited in some novels in Qing Dynasty in the beginning of 20th century. Such as:
The author puts: 排 seems to stem from the 排骨 in 排骨麵 served in Shanghai being similar to Western pork chop. (上海「排骨麵」裏的大塊「排骨」近似西菜裏的「豬排」,「排」字的用法似由此而起。)
The pronunciation of 排 in Shanghai local dialect is similar to 扒 in Cantonese, which seems to be the reason that Cantonese also use 牛扒 to mean 牛排. (「排」,上海話讀「bɑ」,廣東人聽成「p’a」,似因此也寫為「扒」。)