Fetching details from the dictionary...

to become stronger; to grow in intensity; to grow violent; to become worse
to invite contributions, etc.; to solicit help, participation, etc.; to recruit (e.g. soldiers)
不安が募る中で冷静な判断を下すのは難しいものだ。
It's difficult to make calm decisions when anxiety is growing.
時間が経つにつれて彼への思いが募っていった。
As time passed, my feelings for him intensified.
寒さが募って外出する気になれない日が続いている。
The cold is intensifying and days continue when I don't feel like going out.