Unfortunetely, you do not appear to be smoking anything - blame DL's fare rules.
I fly DTW-LGA weekly, typically M-Th, or Tu-Fri.
Looking at fares, when you stay for 3 or more night, the fare is consistently $289 r/t. However, when you book for 2 or fewer nights it goes up significantly to $650-$700 plus.
There lies your problem, the fare rules for the cheaper fares appears to have a number of days/night restriction.
This wasn't the case in the NW days, but with DL running the show, they appear to significantly be jacking up last minute, and short-stay business trip style fares. People have complained about similar issues on DTW-DCA.
Granted, I thought it was crazy to be able to get $150 r/t earlier this year on DTW-LGA, and I'm not going to complain at <$300. If it goes up more from there, then I'm going to start having issues.
Spirit and AA have both cut back on the number of flights on DTW-LGA as well.