Please refer ASME B 31.3 -302.3.6(Limits of Calculated Stresses due to Occasional Load)-
The code compliance requires Checking the "Sum of Longitudinal Stresses due to Pressure,weight,other Sustained loads SL AND of the stresses produced by Occasional loads such as Wind,Earth Quake may be as much as 1.33 times Sh.

Also refer section 302.2.4 (Allowances for Pressure-Temp Variations)-Para f(2) which states-
When the variation is self limiting(eg . Due to pressure relieving event)and lasts no more than 50Hr at any one time and not more than 500hr in a year. it is permissible to
--exceed the pressure rating or
--the allowable stress for pressure design at the temp of the increased condition by not more than 20%.

As a stress analyst during the analysis for a Relief Valve Pop Up Case, (which is normally considered occasional),the usual practice is to check for the 1.33 Sh criterion.
Is the 20% criterion per 302.2.4 also to be checked for this Popup case?

Request the community to throw some light on the Basis of these allowances(33 % for Wind,Earthquake per 302.3.6 and 20% per 302.2.4)

Para 302.3.6 addresses the stress component in the longitudinal direction which, in most of cases, is the sum of axial stress due to pressure and bending stress due to sustained loads.
Para 302.2.4 addresses Pressure Design, which is based mainly upon the pressure stress in circumferential direction (hoop stress).
For the popup case both of these two paras will be applicable separately i.e. for pressure design (calculation of pipe wall thickness and component rating) Para 302.2.4 will be applicable while for longitudinal stress check Para 302.3.6 will be applicable

Now why there is differentiation in occasional allowance between the two components of stress, which is unconditional 1.33 w.r.t loading duration for the former case and conditional 1.33 or 1.2 for the later case? Might be because of localized nature of the bending component in the former case.

Paragraph 302.3.6 gives the maximum allowable increase as 33%. Note that this paragraph states " ... MAY BE as much as 1.33 times ...". What the code is saying here is that the allowable can be up to 1.33Sh. The code didn't say it was 1.33Sh. The value 1.33Sh is the maximum allowed, but it could be lower due to other factors.

Paragraph 302.2.4.f.1 defines how the "occasional allowable" is determined, based on the frequency of occurrence or duration of the load. Note that sub-paragraph (b) here states that the allowable is 1.20Sh for loads that occur no more than 50 hr at any one time and no more than 500 hr per year.

This agrees with paragraph 302.2.4.f.2 (which you refer to). I believe that for your situation, the allowable should be 1.20Sh, not 1.33Sh.

In my opinion Para 302.2.4 deals with the allowances for variation of temp. & pressure above design temp. & pressure. It does not say anything about calculated stress of a piping system because of occassional loading.
Whereas 302.3.6 says about the limiting value of calculated stress in a piping system with occassional loading.
Hence these two clauses are entirely different.