>What do those mean
Both of they are the emphasis expressions.
They become like the yell if it reads it aloud.
怖い→怖ええええ
うざい→うぜえええ
>why are they sometimes romanized?
There are two kinds of Japanese input methods.
kowai(input)→こわい(display)→怖い(converted)
This method takes time a little, but it is necessary to memorize only the arrangement of the alphabet.
2. Inputting Kana directly and converting Kanji.
こわい(input&display)→怖い(converted)
This method doesn't take time to this method too much, but it is necessary to memorize two pattern of arrangement of the alphabet and Kana.
http://www9.big.or.jp/~mshinoda/guidance/keyboard.gif
Many people use method of 1.
Therefore, the slang word came often to be input like the Roman alphabet in the chat for which a quick input was necessary.
Because the mark by the alphabet became a visual emphasis, it might have been established as the mark of the slang word of the emphasis expression.
