Balance changes come from different people and from different motivations, and yes it happens rapidly.
Often changes come in response to feedback from players, when data supports the hypothesis. Sky Lobster nerf in response to the days of Lobster-K is an example of a highly demanded change to a unit that was stealing the spotlight almost every clusterpot round. Hover and Locust nerfs have been in response to their domination of certain maps in the 1v1 rotation (the speed meta). Merlin health was dunked because people were tired of losing their armies to it in FFA.
Sometimes changes happen when a unit is under/overachieving its design goals. Raptor combat speed and maneuvering changes make it work better at what it's designed to do, while being careful not to significantly change the unit's "on paper" value or matchup with Trident.
In the case of Imp, we're talking about a bomb unit which walks out of a factory with cheap area cloakers. It is the early game force multiplier from hell and is only balanced by the amount of micro needed to use it effectively. Making Imps immune to each other's stun damage would make it a redundant anti-heavy option for Cloakbots and a less efficient version of Widow in that role.
If you need to counter Detri you are solidly in The Late Game. Send in some Ultimatums and go to town. Send in 10 Phantoms and kill it in six shots. If you didn't know the Detri was coming and you don't already have an army that can kill it then you've already lost The Late Game.
Even if Imps were immune to each other's stun damage it would be cheaper to build a Spiderfac and 11 Widows than the 35 Imps it would take to stun the Detri.
When a unit fails to meet expectations, it can mean the expectations were wrong because the game was misleading. I do see that the Imp's description text suggests "it can paralyze heavy units" which is kind of misleading. This might be worth fixing.