It typically means the stock is tanking. If you know your $10 stock is going down, it looks better to merge it back into $1000 stock; this way, when it tanks, instead of $0.50 shares you get $50 shares - which look more presentable to new investors.
Very roughly speaking, Split -> Stock is so good the company wants to cash in; Buyback -> Stock is still good enough that the company wants it back; Reverse Split -> Stock is tanking and company doesn't want to scare people.
