excel multi-sheets

I am loading excel files with multi sheets into sql server table(s).

For example, there is only one dataflow task, in there, there are four sets of source to destination and each source being a sheet in the excel file.
On observation, I see that most of the time, the dataflow tasks (source to destinations) run in parallel but sometimes I get errors...

My question is could there be any locking of any sort if the data is being imported from multiple sheets into a sql server table?

Thanks