Usually age and inadequate maintenance, but I expect TC is within regulatory requirements. Usually Enbridge is making headlines with pipeline leaks.

There are very reliable testing methods that require shutting down the pipeline, and I understand there are some technologies available too, but both are 'too expensive'.

The position of petroleum producers is that the existing regulations are already too strict and burdensome to justify the capital cost and risk of building new pipelines. They aren't doing themselves any favors and may need to get used to lower margin business.

To me, the growing demand for petrol, the age of the existing pipelines, the relaxed regulation, low popularity of new pipelines, and the relatively low margins in the current market are a recipe for disaster. Populism is only making it worse (drill baby drill vs shut down all the pipelines), such stupidity makes my head hurt.