here are some suggestions for preventing this next time:
1. you could call. your goal is to check the progress of the task and give the employee a chance to respond with questions — a call could accomplish this while allowing both of you to hear each other's tone of voice. since you say at least two of your staff find your emails harsh, this could be the easiest solution.
2. include deadlines when you first make a request. if you don't have time to call, or if time zones make calling a hassle, email can still work if you revise your initial request to include a due date, or, for a bigger project, a due date for an status update. by giving a date, your follow-up won't feel as aggressive and your employee will be less likely to take it personally. deadlines will also help your staff prioritize — perhaps rendering follow-ups unnecessary.
3. be conversational. even if the first email was unchanged, the second email could be written to avoid a blow up. you don't need lots of smiley faces or exclamation points. you just need to write the way you might actually talk. for example, your second email could read: "hi cindy, just following up on the email below — can you let me know how far along the task is? i was hoping we could have this by the meeting on thursday, is that a possibility?"
4. use the passive voice. it doesn't make for great writing, but it can help you avoid sounding accusatory — and it sounds like your employee felt accused.
5. share your train of thought. if i know why you emailed me 37 spreadsheets and asked for me to combine them by tuesday, it allows me to be part of the process rather than feel like a cog being dumped on.