So, to open this up...
Based solely on your descriptions, you did not do anything wrong. Your actions here were entirely impartial. The platform had a known weight limit that was established before the PCs set foot in the room. You had no knowledge of who would step onto that platform, if anyone. And everything was appropriately rolled for, and what happened is what happened. It's not even necessary to make it clear that ground is unstable before someone steps on it (see: Pit Traps). As Icyfire said, it's nice of you to do so for new players, but it's not required.
The problem here seems to stem from a fundamental misunderstanding of how D&D, and Role Playing Games in general, work. It sounds to me like you guys need to have a meeting where you have a clear discussion on what you are expecting from the game, and make sure a couple of things are nice and clear before you continue playing. You may have to put a bit of work in to figure out how best to explain these things to your particular players.
This is a place where a lot of confusion can come up, and I often find it best to draw comparisons to video games in some cases.
You need to find out what your players are expecting from this game.
Are the players expecting a casual romp where they rove through dungeons and kill everything with little effort or risk to their characters? This is clearly not how you are running the game, and it's not how D&D was designed to be run.
Do they understand that, like a video game, bad things are going to happen to their characters from time to time?
Do they understand that, as the DM, it's your job to control the opposition and set up things that can hurt them? And yet, despite this...you're actually on their side? This is a weird thing for players who are used to competitive games to get their heads around. The goal of the DM is to create a challenging and engaging adventure for players to complete, and that means creating threats that can harm their characters. But you're not actually trying to kill them or prevent them from 'winning' the adventure. If you were, you'd just sic the Tarrasque on the first level adventurers...or have them be hit by a meteor...or spontaneously combust.
And one last thing to make sure your players understand...
For the sake of keeping the game moving forward, D&D makes it pretty clear that the DM is the final arbiter of the rules. The DMG explicitly says...
The rules aren’t in charge. You, the DM, are
The rules are a tool that you and the players use to have a good time
In short, the first rule of D&D is that if the DM contradicts a rule, the DM is right (and, if a rule gets in the way of everyone having fun, the DM can declare the rule void). This is an enormous amount of power and, as the DM, it is your responsibility to wield it responsibly. If players disagree with a ruling, they can bring it up...but as DM, you must have the authority to "Make the Call" so the game can continue. That's your Job. And, frankly, the DM has the hardest job by far.
That said, be humble with this power. If you were wrong about something, own up to it and see if you need to make it right.
In short, you need to sit down and make sure you are all on the same page with regards to what you expect out of D&D. And don't just steamroll their opinions, you're all in this together. If your players want plenty of warning before 'something bad' happens...then maybe you can start calling for Perception or Insight checks when a character is about to do something hazardous to their health, or focus your descriptions harder on hazardous things.
And you also need to make sure you all understand how D&D works, particularly with respect to how the DM and Players are meant to interact. It's laid out pretty plainly on page 6 of the Player's Handbook.
- The DM describes the environment
- The Players describe what they want to do
- The DM narrates the results of the adventurer's actions.