Probable reasons this happened:
1) the store cheaped out and either went with compromised used fixtures, or did not use the proper weight rated fixtures
2) the fixture was rated for the correct weight, but was improperly assembled.
3) the fixture was not properly leveled. This takes a little extra effort, but is worth it
4) even if the fixtures were correct and properly installed and levelled, if the other side was completely empty this could happen. Not as likely, but possible.

I used to travel a lot for work, and I have cooked a lot of ramen noodles in hotel coffee pots over the years. Not exactly haute cuisine, but it's cheap, and it works.