deadlock caused by a cyle of locks ? unprobable event causing it or just a false intuition
If a thread takes the Couple 5, the thread taking the couple 4 will lock D and wait, etc... to Couple 2. 


Couple 1 tries to start and is waiting on A, couple 4 tries to start but is also waiting on A.

After couple 5 finishes one of the waiting couples will be able to lock A. If it is couple 1 then it will need to lock B before it can continue, however that is held by couple 2 and that is waiting on couple 2 which is waiting on couple 3 etc. until couple 4 which is now waiting on couple 1. There is no way to make progress $\implies$ deadlock.

It's important to keep in mind that between locking one word and locking the next a process can be interrupted or another can simple be faster and lock the second word.

You can avoid the lock by always locking the words in alphabetical order, that way couple 4 will lock A before it will try to lock D and couple 3 is then able to make forward progress and a deadlock is impossible.