Try this:
data have;
input RATE;
datalines;
0.16
0.1
0.07
0.08
0.12
0.1
0.08
0.429
;
run;

/*Calculate Cum_Rate*/
data cum;
        set have;
        LABEL='N';
        retain CUM_RATE 0;
        if CUM_RATE>=0.2 or RATE>=0.2 then CUM_RATE=0;
        if _n_=1 then CUM_RATE=RATE;
        else CUM_RATE=CUM_RATE+RATE;
        if CUM_RATE>=0.2 then LABEL='Y';
        n=_n_;
run;       

/*One observation down shift to carry over the RATE on the next observation */
data temp;
        set cum (firstobs=2);
        n=n-1;
        keep n RATE;
        rename RATE=RATE_NEXT;
run;

/*Merge back to apply condition of Next Rate>=0.2 */
data want;
        merge cum(in=x) temp;
        by n;
        if x;
        if RATE_NEXT>=0.2 then LABEL='Y';
        keep RATE LABEL CUM_RATE;
run;

temp数据集中要把CUM_RATE和LABEL drop掉，不然最后一步merge的时候会把cum数据集中的结果覆盖