What might cause a SQL log backup to throw this error??

DESCRIPTION:    The operating system returned the error '32(The process cannot access the file because it is being used by another process.)' while attempting 'DeleteFile' on '\\mls-storage1\backup\sql4_backup\SQL4\UNI_MOUN1309\LOG\SQL4_UNI_MOUN1309_LOG_20110705_210005.trn' at 'storage.cpp'(875).

I use Ola Hallengren's backup scripts; he suggested I inquire to the forums. I am particularly puzzled why the 'storage.cpp' mention since the log backup script is a SQL script.

Thank you, Tom