Hello,
Because categories take 2 lines on the mobile device http://prntscr.com/swslsk
Either choose the primary category in the product options http://prntscr.com/swsmjt or add min-height for the categories block using below code
@media only screen and (max-width:480px){
.content-product .products-page-cats {
min-height: 5ex;
line-height: 2.5ex;
}
}
Or disable the categories for the shop grid.
Choose the most suitable for you way.
Regards