could be or did you slacken the belt off if so you my have done the alternater up to tight or if you had removed it totaly check you have done all the bolts up and that the botom ones are'nt lose as the alternator will move on the piss and it will be pulling the belt to one side strain in it

just worth a check as i've done the same thing in the pasted left the bottom one slightly lose and it brook the casing