How does 行 evolved to mean industry?
My intuitive guess was it has something to do with 行会. So I looked it up and there it is:
from baidu baike
(although the entry is about 行会 and not 行, so everything would technically still be based on my original assumption, which isn't that bad of a guess from a native speaker's perspective):


  行会产生于隋唐。唐代工商业组织大都称“行”，源于街巷上的贩卖摊商，往往一条街上开设的都是同类的店铺，故称“行”，如“织锦行”、“金银行”等。到了宋代，行会组织更加发展。


So it turns out that during Sui/Tang dynasty, it often happened to occur that along a certain street, there were exclusively shops doing the same kind of business, and therefore the 行 (line).