That seems to be a common occurrence when placing crops and sometimes workstations. Not sure what causes it (bug likely), but as long as the crops have at least 2 3 empty blocks (I always forget how tall corn is) above and are within ~7 blocks (I think) of open sky, they should grow fine.
I have heard of cases where fully-grown crops will cause a collapse if the underlaying structure isn't supportive enough as (as far as I'm aware and they haven't changed anything) fully-grown crops actually have more mass/weight. I've never had it happen personally, though.