Unusual and unwanted Excel behaviour when inserting while clipboard holds data

Can anyone explain why it is that if I have data on the clipboard (copied from one workbook or worksheet to different workbook/sheet) and have pasted that clipboard info and saved the file with the newly inserted data then try to insert a column or a row Excel insists upon creating the column or row BUT then fills EVERY possible unfilled cell in the column (2^20 rows) or row (2^14 columns)! This often - if it succeeds takes up to 2 hours o fill (ans IK do NOT want the cells filled anyway. More often than not, Excel fries its wee little brain and locks the entire system up requiring a restart!

Any ideas why this happens and more importantly, how to STOP it from doing this?
Thanks in advance to all the Excel gurus!

That is exactly what I did!
What files would you require? The source and the destination? BOLTHY are QUITE LARGE and one contains at least 9 VERY large worksheets (historical and current foreign exchange rates from several banks).

I SOMETIMES prevents this massive fill from taking place but it's inconsistent if it will stop it. Besides, I never had to do that before!

OK that explains it. M?ust be a way to stop it!
Thanks for the insight.