Does anyone here know the reason why only half of the pic was exposed?

Maybe the film was exposed to some sort of light when it was being removed from the cartridge?

Did you develop it or did you take it to a developing lab?

Two things can explain that, especially if you use digital or film SLR's. First, it's possible that the reflex mirror is not swinging fully when you hit the shutter. Or something is blocking half of the mirror. Second, it's possible that a shutter leaf of your lens is not opening properly, thereby blocking half of your shot.

Where were these half exposed shots on the roll? It is very common for the first couple of photos you take and the last couple of photos you take on a roll to not be fully exposed. But if they were sporadically throughout the whole roll, then it could be something wrong with your camera or there was a problem in the lab when developing the photos.

Very, very few slr cameras have leaf shutters, and the image you posted does not have the look of a jammed shutter;leaf, anyway. Your SLR will almost certainly have a focal plane shutter, and if it was jamming you would expect the shadow line to be parallel with either the top edge or the frame side, depending on whether it is a vertical or horizontal shutter.

Same is true for a jamming mirror, and anyway, you would have seen that in the viewfinder. Also, because the shutter is so close to the film, you would expect a sharper terminator (shadow edge).

I would suspect film transport, but it looks even more like a simpler problem, like a camera strap or piece of clothing blowing across the lens at the time of the exposure, which you may not have seen because the mirror flips up as you press the shutter release. Actually, I have seen a roll of film that looked a lot like this; the culprit turned out to be a baseball cap throwing a shadow over part of the lens. Only happened on bright days, and only on some frames...

Looking from the pic uploaded it seems the problem is in the lab, the roll got exposed, as you said the pics are in the middle of a roll. The pics in the front and back might have been in good condition. I mean properly exposed. The four shots got exposed to light during the processing. From my experience I don't think it is either the shutter or mirror problem. If so the entire roll might have got the frames half exposed and half unexposed. Since the shot you enclosed is a day shot hopefully there is no problem of flash syncronization. Check up with another roll and let me know.

Were you using flash on those photos? If so, then the simplest explanation is that your shutter speed was higher than your flash synchronisation speed. That means that when the shutter trips, and the flash fired, the leaf aperture of the shutter was across half the image.