“The bull walks up the stairs and thee bear jumps out the window” means that a market drops (“bear”) faster than it rises (“bull”). The saying has been cited in print since at least 2000.
       
A similar saying is: “Up a staircase, down an elevator.”
 
   
Google Books
Rich Dad’s Guide to Investing:
What the rich invest in that the poor and middle class do not!
By Robert T Kiyosaki and Sharon L Lechter
New York, NY: Warner Books
2000
Pg. 197:
As rich dad often said about technical investors, “The bull comes up by the stairs and the bear goes out the window.” A bull market will rise slowly. but when it crashes. the market is like a bear going out the window.
 
Google Books
DIY Portfolio Management
By Lyle Wilkinson
Wailuku, HI: Select Publishing
2003
Pg. 165: 
Remember the Wall Street wisdom: “Bulls climb the stairs, bears jump out the window.”
   
Google Books
Advanced Hypnotherapy for Professionals
By Steve G. Jones, M.Ed.
Lulu Publishing
2007
Pg. 66:
Bulls go up the stairs and bears go out the window (a metaphor explaining that stock prices rise slowly and drop quickly).
       
Airline Pilot Central Forums
FreightDawgyDog
02-26-2007, 12:36 PM
“Making SWA $$$ again? Like you said, it might take a few years. Remember the phrase: “The bull walks up the stairs, but the bear jumps out the window.” With nearly $40k cut for everyone, I bet the bull will take it’s ol’ sweet time to climb to the tenth floor…
   
Optionetics Discussion Boards
Wink (13 posts) wrote on 9/28/07 7:52 PM
(...)
I prefer Bear Call Spreads (at least 1 standard deviation OTM) because “bulls walk up the stairs, bears jump out the window.”
 
Google Books
Getting Started in Candlestick Charting
By Tina Logan
Hoboken, NJ: John Wiley & Sons
2008
Pg. 215:
I once heard it said that “the bull walks up the stairway but the bear jumps out the window.” This is a clever way of saying that prices tend to fall faster than they rise.
     
Weekly Technical Commentary
Wednesday, September 24, 2008
TODAY’S BREAKOUTS & BREAKDOWNS
(...)
Ever hear that phrase “Bulls walk up the stairs but bears jump out the window”? Six years worth of gains gone in within one year. Please, don’t try to find the bottom unless it’s making higher lows.
   
ShareChat
contango
Thu, 09 October 2008 11:35
The bull walks up the stairs and the bear jumps out the window.
   
A. J. Brown’s Option Profits Success System Review
7 Devastating Mistakes Most Option Traders Make
October 4th, 2009
(...)
Believe it or not, you can often make twice the ROI on short trades than long trades. As I’m fond of saying, the bull walks up the stairs, the bear jumps out the window, which is just another way of saying that the market drops faster than it rises.
   
The Traders’ Club
Tuesday, February 9, 2010
INCREDIBLE PRICE ACTION IN COPPER
Traders take note: Copper just “jumped out the window.”

There’s a classic Wall Street saying that goes, “The bull walks up the stairs and the bear jumps out the window.” Bull markets tend to march higher and higher as money required to boost prices steadily flows in. When at last there are no new buyers, investors can see months and years of gains erased by a fast decline… We’re seeing this phenomenon at work in the vital commodity we call Dr. Copper.