to reel in credit lines

It means to reduce or eliminate. My personal opinion is that this metaphorical usage is somewhat strained, and clumsy. It is taken from the sport of fishing with a line and bait. When one has hooked a fish using a fishing rod with a line, to bring the fish to shore or into one's boat is called "to reel it in," because most fishing rods have a reel to wind up the fishing line. As i say, it is used here in a clumsy fashion.

How do you this, Setanta?

Whatever the source, if there even is actually one, it's perfectly apt. It describes bankers acting very quickly, taking back the lines of credit, just as a fisherman does or as a sailor does to reel in line.