Guides or hairs generated using the Strand Multiplier modifier will stretch when combed.



This happens because all of the generated strands by multiplier have the same root positions by default. If the Surface Tracking setting in Edit Guides modifier is set to Surface(which is the default) the changes made by the comb brush will be applied multiple times in the same place.



In Strand Multiplier modifier use the Spread Root and Fluff Root parameters to spread the roots apart.


Solution 2 

In Edit Guides set Change Tracking to Strand.

Missing Something? Let us know if this page needs more information about the topic.