。
I'm afraid trouble might be brewing.
Literal
Trouble [subject-が] happens [nominalizer-の-ではないかと] I [topic-は] am-fearing.
~のではないかと思う/恐れる is a stock frame for 'I think/fear that X might be the case.' Built from ~のではない ('it is not that X') in a question + quotative と — 'I worry that isn't it the case that X?' Very common in careful, hedged speech. 恐れる ('to fear') is more literary than 怖がる; the ~ている form suggests an ongoing state of worry.