like yours they just burned good.
Even professionally-made black powder will only burn if it's not enclosed. The trick to getting it to explode is to seal it tightly, *then* light it off. No, the lack of air won't slow it down any, the potassium nitrate is an oxidizer, it supplies the necessary oxygen for the reaction.
When enclosed, the first bit of powder to burn greatly increases the pressure in the package, which greatly increases the burn rate of the remaining powder, which exponentially increases the pressure which exponentially increases the burn rate (rinse, repeat). Result: *BOOM*, as all of the powder ignites almost simultaneously and produces a shock wave of pressure that blows apart the container.
This is why firecrackers go "bang", the small amount of powder in them is tightly wrapped in layers of paper -- try peeling one open sometime and then burning the enclosed powder in the open -- it just burns, no bang.
A good wrapper for homemade gunpowder is the cardboard tube on a cheap wire hanger (the kind made for hanging pants). Cut off a few-inch section, fill with powder, pack both ends with paper wadding, then seal with epoxy (don't forget to run a fuse into it before sealing). This will be a home version of the classic "M-80" fireworks.
The tighter the seal, the better the bang -- too weak a seal will blow open early and release the pressure before it builds to a peak.
Pipe bombs work on the same principle, but I do *NOT* recommend fiddling with them. Not only are they large enough to cause serious damage (and shrapnel), but the very act of screwing on the threaded caps can set off the contents and ruin your whole day.
Paper and glue are more than good enough for the all-American practice of playing with fire and loud noises.