Im breaking my strings at the top ?

i play with a dunlop m fil 300. i string my racket with dunlop explosives. so far theyve been broken twice but not in the sweetspot. my friend broke them both times. once when retuning a hard serve and once while serving. both times they broke at the upper right hand corner. those strings broke like in a month and then in a week. theyre supposed so last really long. and break in the center. so wats wrong. strings arent supposed to break even on mishits