How did a killing at a Sikh temple lead to Canada and India expelling each other’s diplomats?

Ottawa has identified India's top diplomat in Canada as a person of interest in an assassination plot

Relations between India and Canada have reached a low point as the countries expel each other’s top diplomats over an ongoing dispute about the killing of a Sikh activist in Canada.

Canada says it had identified India’s top diplomat in the country as a person of interest in an assassination plot and expelled him and five other diplomats Monday. India has rejected the accusations as absurd, and its foreign ministry said it was expelling Canada’s acting high commissioner and five other diplomats in response.