OK, 5 gallons (finished beer volume) is normally 52 12 oz bottles so you are a little under that but that alone is not enough.
You also mention you were still getting airlock activity so likely the beer was not 100% finished. Still, unless it was stalled fermentation, that alone is not your problem.
You likely over-primed depending on the style of the beer if you used the supplied kit volume of corn sugar unless you buy from a company that taylors that to the beer style. Still, not enough for gushers.
The beer was young in the bottle and warm. The CO2 is likely not in solution do to both of these things.
If you add all this up, you likely have a number of small things that contributed. Will a beer in the fridge for 24-48 hours, popt the top over the sink. If it lightly foams out the top but does not gusher on you, it may also have the contributing factors of enough adjuncts that promote good head (wheat, carapils, etc) that it appears worse than it is. Immediately pouging slowly and carefully in to a British Pint or similar sized glass may reveal a perfectly good beer under a mountain of rocky head. This happenned to me the first time I added 10% wheat in addition to my normal 5% carapils in an already Munich Malt heavy beer.