why did archer...

sever his contract with rin in fate hollow/ataraxia? this never got explained which is why it confuses me. rin also seemed to be upset by his choice as shirou even says rin was in a bad mood & pointed out how they were still connected cause they kept acting similarly (investigating the same spots on the bridge at a high place). rin gets annoyed everytime shirou asks about archer. but it also doesn't seem she knows archer's identity because she never figured out how archer & shirou could see those monsters?

i assumed it was because at the end of ubw, he told rin he didn't have the right to be her servant amymore so it was lingering guilt but i'm not sure.

i also wonder if shirou taking care of a sick rin was apart of that because archer would've also easily cared for her.