It now seems clear why David Sánchez resigned. This news report in La Patria (an Oruro newspaper) explains: Sánchez tried, in vain, to win an audience w/ Evo to ask him not to go forward w/ a planned mobilization of as many as 10,000 cocaleros (from La Paz & other areas) will march to Sucre on September 10. Sánchez feared that this might lead to violence. In the 1960s, René Barrientos (ironically, rabidly anti-communist) & other military dictators often used such tactics to subdue regime critics. These were the days of the so-called “military-campesino” pact.
This could be the straw that breaks the camel's back...amazing that Sanchez went to La Paz to talk to Evo and he refused to see him, pawning him off on subordinates.