Why does the English muffins package say to use a fork or fingers instead of a knife to open them

Perhaps I'm stating the obvious, but is your oven rack perfectly level? Muffin mix initially has low enough viscosity that it can be influenced by gravity as it gradually rises. Even a small inclination of the tray/rack is likely to cause this.

This table comes from the front of the muffin section in Bread, by Beth Hensperger:

Muffin size          Baking time     Yield
Mini/gem (1 5/8")    10-15 minutes   18-20
Regular (2 3/4")     20-25 minutes   9-10
Oversized (3 1/4")   25-30 minutes   6-7
Muffin cake (8-9")   55-65 minutes   1

The baking times are for 375-400°F; most recipes will fall around that range. Note that the yields don't necessarily match up to typical pan sizes; for example, if you take a recipe meant for 12 regular muffins, you'll probably have more batter than you need for a pan of 6 oversized muffins. Perhaps this is your problem - you could be overfilling. If even after scaling the volumes appropriately, you still have problems, you could try reducing the temperature by 25°F and increasing the baking time.

("Muffin cake" refers to baking in an 8-9" pan of some sort - the author says that you can get away with it for any muffin recipe, but I've never tried it!)