Cuba has been a long time ally of Russia, and Russia has been aggressively trying to reassert their power on the world stage.
The other thing is, that a large part of the spy game is that intelligence agents often work out of embassies. The way the game is played is that as long as intelligence personnel stay in their lane, there's no trouble. The US has spies working out of embassies around the world, and foreign intelligence services have spies working in embassies in DC.
But for an adversary like Russia, maybe they would try to attack people on diplomatic passports if they knew there were some way to make it untraceable or covert. Cuba might do the same if they felt they could thwart US spying.
Note that there was also an alleged incident like this at the US embassy in Uzbekistan:
http://www.dailymail.co.uk/news/article-5126383/US-embassy-w...
State Dept. denies it, but if there is an intelligence angle to this, denials would be standard.