Question

why precipitate is formed only in some of the reactions like double displacement reaction

Answer 1

Precipitation Reactions. Precipitation reactions occur when cations and anions in aqueous solution combine to form an insoluble ionic solid called a precipitate. Whether or not such a reaction occurs can be determined by using the solubility rules for common ionic solids.
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Answer 2

In a double displacement reaction, the anions and cations exchange their partners in an aqueous solution resulting in a solid insoluble substance called the precipitate.
For example:
Pb(NO3)2 + 2KI ⇒ PbI2 +2KNO3 
where Lead iodide is the precipitate.