Why was Jesus crucified in the middle? We read at Mark 15:27 

"And with him they crucified two bandits, one on his right and one on his left.""

One wishes to know as to whether it had been pre-determined by the authority say, Pilate, that Jesus be crucified in between two thieves, or was it by sheer chance that the soldiers gave him the middle slot. Does the Catholic Church elaborate on the position of the Cross of Jesus at Calvary, in order to attribute the fulfillment of prophesies ?
Why was Jesus crucified in the middle?

It was a decision made by either the the Roman Governor Pilate or the Roman executioners. 

The Catholic Church has no official stand as to why Jesus was crucified in the middle. However, it does make sense in a logical point of view that the Romans would have placed a criminal who was deemed more dangerous or guilty in the middle. (The place of honour so to speak.) The Catholic Encyclopedia makes an allusion to this possibility:


  The cross on which Jesus Christ was nailed was of the kind known as immensa, which means that the vertical trunk extended a certain height above the transverse beam; it was thus higher than the crosses of the two thieves, his crime being judged a graver one, according to St. John Chrysostom (Homil. v, c. i., on I Corinth.). The earliest Christian Fathers who speak of the Cross describe it as thus constructed. We gather as much from St. Matthew (27:37), where he tells us that the titulus, or inscription containing the cause of His death, was placed, "over", the head of Jesus Christ (cf. Luke 23:38; John 19:19). St. Irenæus (Adv. Haer., II, xxiv) says that the Cross had five extremities: two in its length, two in its breadth, and the fifth a projection (habitus) in the middle — "Fines et summitates habet quinque, duas in longitudine, duas in latitudine, unam in medio". St. Augustine agrees with him: "Erat latitudo in qua porrectæ sunt manus longitudo a terrâ surgens, in quâ erat corpus infixum; altitudo ab illo divexo ligno sursum quod imminet" (Enarration on Psalm 103; Serm. i, 44) and in other passages quoted by Zöckler (Das Kreuz, 1875, pp. 430, 431).
  
  The historical narrative of the Passion and Crucifixion of Jesus Christ, as found in the Four Gospels, agrees exactly with all we have set down above concerning this form of punishment. Jesus Christ was condemned for the crime of sedition and tumult, as were also some of the Apostles (Malalas, "Chronogr.", X, p. 256). His Crucifixion was preceded by the Scourging. - Archæology of the Cross and Crucifix


This question is somewhat explored by Jimmy Akin’s (Catholic Answers) in the YouTube video: Why Was Jesus Crucified in Between the Two Thieves?