Was the Defcon raised when Ronald Reagan was shot? If so, why?
Richard V. Allen’s eyewitness account of that day: link
Near as I can tell from reading the link below the DEFCON status was not changed but they increased alert readiness withing the bounds of DEFCON-5. Naturally the President being shot might be a good time for an enemy to make a pre-emptive strike while confusion reigns and the US military might not respond as fast as it otherwise might. As such pushing the DEFCON level up might make sense to allow for the lag involved in having te President out of the loop.
Damn you Scrivener! That’ll teach me to write before posting so I don’t get beat to the punch! 
If they had done so, it would be obvious why - this was during the cold war. An assassination could be an attempt to chop off the head of state so there would be confusion if a nuclear first strike was ordered. Or, it could result in an opportunistic move by one’s enemies on an ally.
It was a dangerous time. An assassination attempt is very serious. Until you know exactly what was going on and who did it, you don’t take chances.
They didn’t have to change DEFCON status. Al Haig was in charge! No confusion here, nosiree…