From my understanding of the Canada situation, it may have been motivated by less access to vaccines initially. The US did very well in terms of getting lots of vaccines soon (https://ourworldindata.org/covid-vaccinations) while Canada took about 4 months after the US to really get going. Canada may have been more desperate to prevent Covid (or have their numbers stop lagging the US), and thus been less risk-adverse.
This argument does not work for the UK, as they have been ahead of the US the whole time.
https://www.msn.com/en-ca/news/canada/vaccine-panel-says-canada-can-delay-second-dose-of-covid-19-vaccine-if-shortage/ar-BB1cIJaG This article cites the decision being partly justified by limited supplies and how bad things were.