Model 3 deliveries and production are not an issue, but the drop in S/X sales is more than expected.

Most likely two factors: many sales were pushed up in Q4 because of the drop in federal tax credit, and the Model 3 is just too good and stealing sales from the S. Actually all the moves in Q1 seem to confirm this (removal of the low spec S and lowering prices of S/X). The rumors of a refresh also probably slowed down sales a bit. Maybe some are holding out for the Y too, or for other manufacturers. Also Q1 is always seasonally lower sales and the overall market is softening.

The key is going Q2 financials: they will need to show the effects of cost reductions as well as a rebound in sales. Q1 is already written off as being ugly. The stock will trade close (but seemingly above) the low of the last year so investors are not alarmed (yet).

This is a pretty good summary:

https://apple.news/APgEn7lRxT-St34Ky64CR_Q