I have seen that many rockets jettison their payload fairing at an altitude where the aerothermal heatflux is leass than or equal to 1135 W/m^2. What is the reason behind considering this particular number. Anymore information on this will be appreciated.
It appears that the original value was 0.1 BTU/ft2-sec, which you can see listed in some documents, for example this is from a Boeing information sheet for a Delta II launch in September 2007:
This value can also be seen on page 23 of an SLS Mission Planners Guide from 2014:
0.1 BTU/ft2-sec converted to W/m2 is 1135.65. In another section of the Delta II document they list both units:
In theory the original calculated value could have been 1135 W/m2 and this was later converted to BTU/ft2-sec. But since 1135 W/m2 converted to BTU/ft2-sec is 0.0998 this would be quite coincidental. Also since the value seems to go back to the early days of satellite launches, the U.S. unit might be more likely as the original (obviously BTU itself originated as a British unit).
0.1 is presumably a rounded number which was based on test or calculation results that fell within a certain range, and the rounded value eventually became the standard. Perhaps comparable to the selection of 100 km as the limit for space, or NASA's designation of 400,000 feet for entry interface, both of which are based on empirical data but are also rounded standards.
Describing the Karman line as 62 miles can give an impression of preciseness that isn't actually there. The same with entry interface as 122 km, and perhaps heat flux as 1135 W/m2.
How the 0.1 value was originally arrived at might be harder to determine since it seems to have been in use as a standard for several decades. But it is apparently a pretty conservative value since it (and the 1135 equivalent) is so commonly used and considered a safe amount of heat flux for satellites.