I had my bathroom done by a contractor about three years ago. I have 2" x 4" subway tiles surrounding my bathtub.

I have a long vertical crack running the length of the tiles ( 4-5' long) in the middle of the bathtub. The water is on the south side; the crack is on the west side.

I know that everything will have to be gutted to get to the root of the problem. I assume that it has something to do with the joint of the backer board since it is almost a perfect line.

I did not witness the installation so I am not sure of the particulars. But what would be some of the causes of cracks like this and what needs to be done to make sure this does not happen again?

I would divide possible fault between the work done and the general structure of the house.

Some reasons: no expansion joint between backerboards. Hardie requires 1/8" gap. IF that's what he used.
Or 1/4" thick board instead of 1/2".
Or no gap at first row of tiles at the bottom (tub rises and sinks enough when full of water to put a strain on the framing).
Or 2 or 3 of the above.

As for your house, describe it's age and what is above and below the tub area. A sagging joist could cause a crack, or a leak from the roof, or the exhaust fan...

If I had to guess, I'd say it's the lack of expansion joint. I think a lot of people think Hardie board doesn't expand - it does. It has wood fiber in it. You could remove one tile and look. The person who did the job is responsible if that's it.

Whenever you get quotes, demand a description of materials and methods.