Marcus Irving had unnerved Marty, by telling him that the other versions of himself from different timelines still exist in alternate dimensions.
Marty wrote the letter to Doc to get his help, but when Doc arrived, Marcus used this to his advantage. While Doc and Marty were talking, Marcus secretly removed the tires from the DeLorean time machine, and used Doc's flux capacitor to solve his problem with flux energy, which was the final step needed to allow his Yugo time machine to travel through time.
April 15, 1986. 7:40 am.
I need help, Doc — This might be the most important dangerous thing
I'vewe've ever done.
Just meet me at your secret lab as soon as you get this.
- Back to the Future (IDW Publishing)