Could be due to growing tensions looming over state of H1B visas in US which may be driving many people to consider Canada as a backup. Starting January all IELTS centres in US were booked till March/April, if there are still no seats now that means people are still applying in large numbers.
Also last ITA's list of 456 may also be due to large influx of applicants currently in USA who may have got ECA and IELTS done by March.