It is often caused by a central girder in the basement dropping due to rot at the supports but it could be due to any structural settlement like footings, joist bearing points, etc.

It is also often due to modifications to the structure not properly designed. Plumbers are notorious for cutting joists.

Hire a professional builder or engineer to look at it ASAP.

Clancyb,
Older houses shift over time, naturally developing cracks in the walls as the plaster breaks loose from the wooden lath behind it. The only way to fix the problem is to reattach the plaster to the lath behind it. Plaster repair professionals generally spend enormous amounts of time reattaching the plaster with hundreds of screws and plaster washers, then covering these screws with mesh tape. The process is tedious and, if not done properly, leaves bumps in the wall.

Luckily, there is a new method for repairing cracked plaster walls. The method uses an special adhesive, rather than screws, to effectively 'glue' the plaster back in place. It's been featured by Tom Silva on 'This Old House' (see the Plaster Repair Video) and makes plaster repair much easier and faster.

For more info check out www.plastermagic.com

Here is a link that might be useful: Plaster Repair Adhesive

The adhesive can only fill gaps, it will not correct any bulges that have show up.

ezra, the OP has plaster over wire lath. There's nothing for any glue to reattach the plaster to.
OP: you have plaster from the generation where the very hardest surface was executed. It's harder, so it's also more brittle. It seems like something is afoot in the framing to allow this hard plaster to fracture in the way you describe. If, OTOH, the white coat was separating from the brown coat, that would indicate a water problem.
Casey