Why does my LED blink when too much voltage is applied?
Analogy


I rev my car which has a 6000 RPM redline to 15000 RPM, 

It suddenly loses power for a few seconds then climbs back to 15000 RPM again and repeats the cycle.

Why does it do this? 


ie You are driving the LED and the Arduino outside their "envelopes".
Both are competing to be the first to die.
They are politely telling you this.
Hope that the LED wins (as they usually cost less than Arduinos). 

The internal drive transistor for the pin may be overheating and going off for a quiet cuppa before trying again.  

As efox29 suggests - the Arduino could be resetting.

Whatever is happening, as what you are doing is outside specification for both components there is no certainty that the action you see will be well described by a data sheet.

Don't do it.