Why do we confuse languages?

After taking a French exam today, I had a conversation with some of my classmates, and for some reason, words kept coming out in French! I noticed the same thing was happening to a few people – for example we were all saying “télé-réalité” instead of “reality tv”. Of course I realise the words are similar but is there any reason our brains replaced the English word with that of the French? It was very amusing, but weird to find foreign words coming out of my mouth without my permission!