Why am I being charged US taxes?Updated a year ago

Even as a Canadian e-commerce business, we are obligated to charge, report and remit state-level taxes where our business meets the Economic Nexus threshold. Economic Nexus is based strictly on the sales volume and/or unit count we sell into each US state.

Avalara has a brief article that talks a little more about this here. 

BDO also has a more comprehensive article here.