1. Imtiaz bhai was going on a walk with bhabhi and suddenly a beautiful girl passed by. As usual he turned back to see her & suddenly realized that he was walking with his wife and instantly turned back causing the neck sprain.
2. The other thing can be bhabhi saw him glance at that girl and they had a big fight and you don't need me to tell you who wins when an husband and wife fight. For guys who are unmarried, in such a fight husband gets injured and if he is lucky he just ends up with a possible neck sprain.
3. Other than that, when India lost the world cup game against Bangladesh, the Indian players were not ashamed but Imtiaz bhai took this to heart and kept his head down in shame (being such an avid indian team fan) for a number of days and the result, ya u guessed it right neck sprain.
4. If not the above reason then may be when Sachin was dropped from the Indian squad for the Bangladesh series, he suddely lifted his head up (remember it was always down after India's defeat) and yelled, "Oh God please dont do this to Sachin" and because of the sudden jerk neck sprian!!
5. OR when Aishwarya decided to get married to Abhishek, Imtiaz bhai was shocked and he was shaking his head vigorously saying no no why is this happening. For those guys who dont know Imtiaz bhai when the world was feeling sorry for Aishwarya thinking she could have got a way better match than Abhishek, Imtiaz bhai was the only person in the world who thought that Abhishek would have got a better match than her. Obvioulsy he felt strongly about that and what happens when you shake your head vigorously to disagree with whats happening, oh ya a neck sprain.

 I could only come up with the following options, if you know a better reason then please leave a comment.