I read a study about s+y combinations turning into sh+y. It's because the y is further back in your mouth getting ready for it you pull your tongue back which turns the s into a sh. Basically what the study found was that some people don't ever do it, some people do it sometimes, and some people always do it. This applies to grocery and "this year" but the other things we've been talking about is a dialect difference rather than a sound change (phonological rather than phonetic).