There is a known bug in the use of categories, luckily there is an easy work around. 

The effect of this bug is to prevent navigation back to a parent category when the user pinches to navigate back. 

The work around to this is to enable the side controls for the category.  Once side controls are enabled, the back button can be used for navigating back 

To switch on side controls,  double tap and find the pencil icon on the toolbar, select this then 'Side Controls' and turn them on.  The following screenshot shows this setting in a gallery context, but it works the same way in the home menu page or category page.