It looks like these options were part of a package that were issued to employees in April 2013 and vest over 4 years. If an employee leaves, their options vest immediately and they have 7 days to convert or they lapse. It appears some have lapsed in the past.
In this case, either the exercise has been triggered by a further 2m options vesting in April - which may indicate the shares are being sold and could be concerning - or possibly someone is leaving and has sensibly chosen to exercise rather than allow options to lapse, which won't necessarily mean they are selling...
However, I do think the past practice of distributing significant quantities of options to staff each year is something to watch. Possibly it was justified in the past, as I think some directors may have received options in lieu of payment when times were tough. If this practice continues now they are cashflow positive, it would be something of a red flag to me.
