Half their beards were shaved, like it says in Shmuel II, 10:4. They were ashamed to be among people until their beards returned to look normal. (PF) Why did they not cut the other half? Radak (ibid.) - the custom was not to cut the beard at all, even with a scissors (so there is no Isur of shaving), only the mustache, if not due to pain or Aveilus - "Megulchei Zaken u'Keru'ei Begadim" (Yirmeyah 41:5). A shaved beard is shameful, just the custom is to do so in the lands in which we live [nowadays].