I imagine they did it for one of 2 reasons.Yoiks! I wish they'd do this for the o-matic family - why just the NZap!?
Edit:
To expand slightly on the above: I don't understand why the NZaps were singled out for this buff - their run-while-firing speed was, iirc, already in the top bracket, and arguably they already outclassed the aeros and and o-matics (in the same speed bracket) with their superior range. It would make more sense to apply the buff to aeros and the o-matics, since mobility is more important with shorter range weapons.
Wow you pretty much nailed the reasons why they decided to increase the N-Zap's mobility. Major kudos to you.I imagine they did it for one of 2 reasons.
The first is that the NZap just needed something to give it a stronger niche. Neither NZap really saw any use except for the 89 in Turf War and the 85 in particular was often seen as an inferior Dual Squelcher because it has the same DPH, sub and special, but with less range (and of course, range is VERY important). The only real reason to use the NZap 85 over the Dual Squelcher was its better TTK.
The other is that it could be to help cement its placing in its group. The 52 Gal is basically a stronger but slower Splattershot and the NZap is basically a faster but weaker Splattershot. Yet prior to 2.2.0, the Splattershot and NZap had the same mobility while firing. This would explain why the Dual Squelcher got a mobility buff as well; prior to 2.2.0 it had the same mobility while firing as the Splattershot Pro.