Morrigan's Maze is a randomly generated area that is different every time you go in.
This includes the rooms and layout that make up the area, but also the items that can drop.
There is a pre-defined pool of items that can drop in your particular instance of the maze, the stats on the items themselves are not randomized.
Each instance of Morrigan's Maze is split up into four areas. The first three areas have some trash mobs as well as a "boss" that appears after killing all of the trash mobs. The final area is just one boss.
The items in this pool are usually dropped from the "boss" at the end of each area, but have a small chance of dropping from the trash mobs as well.
The chance of gear and the chance of higher quality gear is determined by the difficulty of your instance, with higher difficulties resulting in a higher chance.
You can also obtain gear from the vendor in the third area by spending the crow coins you collect throughout the maze.