when mixing bleach and ammonia

An example of a single-displacement chemical reaction that has deadly effects is the concoction of ammonia and bleach.

Mixing bleach and ammonia is extremely dangerous, since toxic vapors will be produced. The primary toxic chemical formed by the reaction is chloramine vapor, with a potential for hydrazine formation. Here's a look at the chemical reactions involved in mixing bleach and ammonia, as well as some first aid advice if you accidentally become exposed to a bleach and ammonia mixture.
Chemicals Involved

Note that each and every one of these chemicals is toxic, except for the water and salt.
-- NH3 = ammonia
-- HCl = hydrochloric acid
-- NaOCl = sodium hypochlorite (bleach)
-- Cl = chlorine
-- Cl2 = chlorine gas
-- NH2Cl = chloramine
-- N2H4 = hydrazine
-- NaCl = sodium chloride or salt
-- H2O = water

Likely Chemical Reactions from Mixing Bleach and Ammonia

The bleach decomposes to form hydrochloric acid, which reacts with ammonia to form toxic chloramine fumes:
First the hydrochloric acid is formed:

NaOCl → NaOH + HOCl

HOCl → HCl + O

And then the ammonia and chlorine gas react to form chloramine, which is released as a vapor:

NaOCl + 2HCl → Cl2 + NaCl + H2O

2NH3 + Cl2 → 2NH2Cl

If ammonia is present in excess (which it may or may not be, depending on your mixture), toxic and potentially explosive liquid hydrazine may be formed. While impure hydrazine tends not to explode, it's still toxic, plus it can boil and spray hot toxic liquid.

2NH3 + NaOCl → N2H4 + NaCl + H2O

can anyone explain why it is single displacement and not decomposition which is what I thought it was since the bleach is decomposing to hydrochloric acid.

2NH3(aq) + 2NaCIO(aq) 2NaONH3(aq) + Cl2(g)

Notice how chlorine does partner up with anyone.