Why do yeast ferment Equal sweetener
Your sugar substrate was sucrose. Yeast cells metabolise this by secreting an enzyme, invertase, which splits the disaccharide into glucose and fructose both of which can be fermented by yeast to produce CO2.
According to this site

Equal Original (blue packaging)  is a zero calorie sweetener that contains aspartame and acesulfame potassium as its sweetening ingredients along with a bulking agent.
Equal Original PACKETS: dextrose with maltodextrin, aspartame, acesulfame potassium

This agrees with what you say - that the sweetener that you used contained dextrose - and leaves me very confused. Dextrose is another name for glucose so how can this contain zero calories?
The answer lies in the legal definition of zero calories  = less than 5 calories per serving (presumably per package in this case.) So let's assume that by adding two packs you were adding 8 calories equivalent of glucose.
Calorie content of carbohydrate = 4 calories per gram
Two packs could contain 8 calories = 2 g glucose.
The molecular weight of glucose = 180
therefore 2 g = 2/180 moles
if completely fermented to CO2 each mole -&gt; 6 moles CO2
12/180 moles CO2 = 0.75 litres of CO2.
I conclude that there is enough glucose in two packets of Equal to generate a reasonable amount of CO2.
Yeast cannot ferment maltodextrins. It is possible that aspartame (aspartyl-phenylalanine-methyl ester) could be metabolised, but I think the dextrose/glucose solution is the best explanation.
EDIT:
I meant to say, just to illustrate that this is a significant amount of glucose, standard rich yeast medium is 2 g glucose /100 ml (2 % w/v).
EDIT #2 My back-of-an-envelope calculation has a fundamental error in it, but luckily I am the first to spot it. If glucose is being used fermentatively, then only one third of the carbon will end up in CO2, with the remainder going to ethanol. This means 0.25 litres instead of 0.75 litres. Yeast is so oriented towards fermentation that even in the presence of oxygen the fermentative pathway will dominate.