Sometimes we just feel like to, and sometimes we be living and catching bad/sad moments in our life and your heart gets filled with negative feelings but u didn't talk about it or vent for a long time, maybe cuz u didn't want anyone to see you sad or scared if anyone could judge you, untill ur body cannot handle anymore, even so u may forgot about it ur body was still filled with and wanted to get rid of those feeling and crying does it aswell
So if u want to cry u have the right to, crying isn't being weak :)
And if u don't want to ur parents/siblings to see u, u can cry in your room alone when u feel like they are asleep or when they're outside the house :D!!