As you can see above, the flooded cavern that all the catacombs intersect, lies directly beneath the main entryway of the Asylum, and serves as the launching point for everyone's escape.  The night they escaped, at least some of the prisoners went to the main entryway above ground before retreating below.  It would have been much easier to just go directly below ground rather than stopping in the entryway where many guards would have had a chance to spot them.  Though these inmates are barmy, I do not believe they are stupid.  I believe there must have been a reason for their stopping above ground.

The routes of all six escapees are above.  Eliath (blue), Trolan (white), and The Oldest Barmy (red), all left together down the same tunnel. They decided to split off in separate directions at the first major intersection of tunnels.  The Oldest Barmy began to go a different direction and then mysteriously backtracked to the original cavern. Both the Hound and Malazar were going down the same tunnel (maybe together maybe not) when the Hound also backtracked.  After his encounter with the guards, the Oldest Barmy later followed the new path that the Hound chose. Sougad did not arrive on the scene until much later, and as you can see, he followed Malazar.