Step size warning, why do I get it?

I have the following simple simscape model:

http://imageshack.us/f/266/immaginexge.jpg/

with the ode15t solver and its default settings I get this warning:

Warning: Using a default value of 0.2 for maximum step size. The simulation step size will be equal to or less than this value. You can disable this diagnostic by setting 'Automatic solver parameter selection' diagnostic to 'none' in the Diagnostics page of the configuration parameters dialog

It disappears by using a step size of 0.1. Could anyone let me understand why I get it?

thanks