I have three:
1. I start dating a feeder and they slowly start to make me fat, bake extra, extra helpings at dinner, etc. And do everything possible to hide the fact I am gaining weight including buying bigger clothes occasionally to hide how tight they are getting. After a certain size they do the big reveal they were a feeder the whole time.
2. Being blackmailed into gaining. Pretty simple, basically being told that everyone I know will be contacted about my "disgusting weight gain fetish" unless I start putting on some pounds for them.
3. Uncontrollable weight gain. Basically I have some health problem, or some forced condition that is causing weight gain. Weight gain that cannot be counteracted or lost. I just keep putting it on no matter what.