That shouldn't happen if there is anything planted in the tilled soil, but it will if there is just fertalizer, speed grow, or retaining soil in the tilled spot. If you're sure you planted seeds and not just the fertilizer (most of us have made that mistake at least once on a busy night), then you should report this in the help & bug reports section of this site. Please include information like which console or os you're playing on and which version of Stardew you're using.
Sorry about your crops.