Generally, a 'reasonable excuse' is when some unforeseeable or unusual event beyond somebody's control has prevented that person from filing the return on time. For example:
a failure in the HMRC computer system,a serious illness, disability or serious mental health condition has made the taxpayer incapable of filing the tax return etc.
RTI penalties - HMRC announces a relaxation of penalties
Following feedback from taxpayers and the professional bodies, HMRC has decided to stagger the start of new, automated, in-year late payment and filing penalties, to give HMRC and employers more time to adapt to reporting in real time.