The sink and washer drain pipes were rerouted from their original location and I believe it may have sprung a leak behind the wall. The sink has not been connected yet so I can't comment on regular drainage.

I've used my washer/dryer a few times already and sometimes the water drains fine, and other times, the water backs up the sink drain. Yesterday when I was using the washer/dryer I noticed no water was backing up, I guess we did a good job plugging it up. However an hour later the neighbor downstairs notifies us that her master closet's ceiling is leaking (the mast closet is on the other side of the kitchen wall where the drain pipe originally runs down).

What I can't figure out is where exactly the leak is coming from. There is no evidence of leakage on my floor level and it's hard for me to imagine where further down the drainline the problem exists.

I'm inclined to say that it may be occurring where the rerouting took place, and the reason why I don't see any leakage on my floor is because it's happening behind the walls and going straight down to the level below.

The strange thing is, it doesn't occur all the time so it's not a permanent leak, only when the washer is draining so the pressure plays a role.

I'm wondering if there's a fix on my end I can do to lessen the pressure, perhaps I should let it back up into my sink and then slowly drain away? I'm not sure how feasible it is for a contractor to cut away the closet walls to check on the pipe (rather than cut away at the cabinets and kitchen walls, the neighbor's kitchen ceiling is fine).

Any leads / ideas as to how I should proceed? I called the contractor and he hasn't replied yet.

It it was me, I would cut some access in the walls (if you have to) to get a visual on that rerouting place.

My pressure was\her has not pressure

In the first part you tell me:
The sink has not been connected yet and farther on down the line you say:
perhaps I should let it back up into my sink and then slowly drain away? If you haven't connected the sink and the washer's draining into it then you've located the source of the leak. If you think there's no pressure when your washer discharges you are wrong. Your washer drains with much force,( pressure) and volume. A partial blockage would allow normal drainage to slip bye but would back up when the washer drains. Let me know more about this sink and its connection. Back to you, Tom

Were the lines rerouted behind the drywall? If they were properly cemented, there should be no leak but if they weren't the water pressure from the drain pump on the washer could be causing a leak to open up. You are going to need to take that wall down and look. And you may be responsible for fixing the ceiling downstairs!