Attached is a P3/800mhz with 512mb RAM, 80gb hard drive from a few years back.
Here's a dual core Opteron 275's bandwidth graph:
http://zeus1.kernel.org/bw-zeus1.png
http://zeus1.kernel.org/bw-zeus2.png (its sister machine)
(no, we don't host those)
Depending on what your galleries were doing, you can use a lot of CPU horsepower to serve 15mb/sec. If the pages are completely static, i.e. no php code, no server side includes, etc, then, there should be no problem with reasonable hardware being able to handle 15mb/sec. Even a moderate P4 with a 7200rpm drive would have no problem with 15mb/sec.
If you had a script on the page doing geoiplookups, or were doing some stat tracking on the page by loading some script that was inserting records into mysql, etc and the mysql server wasn't set up right, or there weren't proper indexes on the tables, it is very possible that you could easily overrun the hard drive.
Apache also has a problem with what is called the 'thundering herd'. When you get a massive influx of traffic, depending on how apache is configured, it might not be able to ramp up to the demand and will always be playing catch-up. Or worse, it will try to ramp up to meet demand, and fork processes more quickly than the machine can handle.
Quite a few possible scenarios -- only your current host can tell you what really happened.