Can baking sheets continue to cook toffee?
Toffee cannot continue to cook once it's been poured onto a baking sheet.  There is no heat source at this point and therefore there can be no increase in temperature.  

All other factors being equal, I might suggest looking at your ingredients.  Are you using beet sugar this year when you usually use cane sugar?  It might seem like a trivial difference, but beet sugar does caramelize differently from cane sugar, so it's a relevant concern for toffee-making.

However, you might not need to get to the bottom of this mystery.  You know that the temperature you're using with the current ingredients and equipment is burning the toffee.  You could try cooking the next patch to 5-10 degrees less than your previous attempts so that it won't burn.  I'm pretty sure that 290 degree toffee will be just as crunchy and lovely as 300 degree toffee.