it's not usually, there are many rules for plural nouns.

it's not true > when u want feminin plural > u use the plural feminin of the verb
> when u want masculin > u use the plural masculin of the verb
> there is the irrigular plural > جمع التكسير

I don't understand your example, but anyway

The rule applies when referring to plural non-human objects .. I think.
There is no "reason" for it that I know, it's just that!!

Fine, we use feminine verb in non-human plural, like تسير القطارات بسرعة .. where قطار is masculine word, but in plural form it takes feminine verb (تسير) .. This is the only case that masculine words take feminine verb, thereby if the word is human word (عاقل) then just follow the masculine with masculine and feminine with feminine, like يزرع الفلاحون الحقل
Hope it is clear enough,, you can learn more about that in this website: www.arabicpod.net