twisted toilet - cause?

Hello there. I just had tenants move out of a rental unit, and discovered that the toilet is twisted. The bolts are still showing through the toilet's bolt holes, but viewed from above, one side of the toilet is about 1/2" from the wall where the other side is about 1 1/2"). The vinyl tiles under the toilet area are loose, and the subfloor is damp, so presumably the wax seal is broken.

My question is, how could this happen? Could it be due to the fact that both my tenants were quite overweight? I estimate that each of them probably weighs about 250 lbs. I'm wondering if weight over a certain amount could be enough to misalign the toilet (maybe that's part of the reason that the "Big John" toilets have been developed). They never said anything to me about problems with the toilet, or the fact it was misaligned (which is obvious, you can't not notice).

Thanks in advance for any thoughts or advice.

The flange many not anchored, or a bolt could have come out of the slot. NOt all flanges are particularly strong, and may not hold things in place well. If the anchor bolts aren't a good fit for the slot, it may not take much to dislodge them. It's a good idea to put some caulk around the front of the toilet after it is set to help anchor it and keep crud from getting underneath, but you don't want to caulk all around so you have a chance to notice a leak.

You need to pull the toilet up and inspect the flange. I agree with Jim, the flange is either broken or the bolts have twisted out of their slots. Once the toilet shifted position, the seal was broken. The may be in part because of the size of the users, but a toilet that is properly installed should never shift.