This is just the opinion of your dictionary. According to the KRADFILE, 出 has these radicals:
# K R A D F I L E
# Copyright 2001/2005 Michael Raine, James Breen and the Electronic
# Dictionary Research & Development Group at Monash University.
# See: http://www.csse.monash.edu.au/groups/edrdg/licence.html
# for permissions for use and redistribution.
[ ... big snip ... ]
出 : ｜ 山
[ ... bigger snip ... ]
So, "vertical stick" and mountain are the radicals according to this source, which is located here: http://www.csse.monash.edu.au/~jwb/kradinf.html
Actually, a better terminology is radical plus components. A kanji is understood to have one primary component. This is called the radical. The others are components.
So kradfile says ｜ is the radical, and the kanjidic agrees. The latter states that this character has the No. 2 radical. This No. 2 refers to the Kang-Xi 214 radical index.
See: http://en.wikipedia.org/wiki/List_of_Kangxi_radicals . Here you can learn the name of that No. 2 stick in Chinese and Japanese.
Wictionary assigns a different radical from Kanjidic: http://en.wiktionary.org/wiki/%E5%87%BA . They cite Radical No. 17, 凵 . This opinion is justifiable. When we draw the kanji, we make that shape first, then the vertical line, and then the bottom box.
Offline source: Jack Halpern's The Kodansha Kanji Learner's Dictionary sides with Wictionary: it assigns Radical 17 also.
Unicode Unihan data: http://www.unicode.org/cgi-bin/GetUnihanData.pl?codepoint=51FA
Radical 17, again.
However, 冂 as a radical or component of 出? Nonsensical. Perhaps a typo.