This kind of metonymy is very common in Latin.
For a simple example, vir mortuus is literally "a dead man" but can also mean "the death of a man".
This is somewhat similar to how summus mons can be "the highest mountain" and "the peak (= the highest part of a mountain)".
The point is that reading very literally can ...
The ea (= eā) modifies causa, using the very common adjective–preposition–object of preposition arrangement: 'for this reason.'
The forte is from the noun fors, 'chance' (not the adjective fortis, 'strong, brave'); so the ablative/adverbial form means 'by chance.'