As I mentioned earlier, we don't collect data on the reason, but I can comment that the average duration of EI sickness benefits in 2016-17, the last fiscal year, was 10 weeks. However, we also know that approximately 37% of all EI sickness claimants do exhaust, or in other words use up, all 15 weeks of benefits available to them.
On October 16th, 2018. See this statement in context.