"The most common reason they gave for their departures was workplace mistreatment."
Motherhood is one factor, but I hesitate to go there first because there is still such a problem with harassment in tech.
Congratulations. You've just demonstrated the anti-male bias OP was implying exists in these types of reports. That statement from TFA applies to both female and male employees who left their job.
If you dig up the actual report, you'll find that men left due to unfairness/mistreatment more than women - 40% vs 31%. You read the general stat and assumed it indicated a problem with how women are treated, when in fact it's men who more often feel they're mistreated.
The actual report makes pretty interesting reading. The stats are all over the place. Women report experiencing or seeing more mistreatment, but reported experiencing stereotyping at roughly the same rate as men (23% vs 24% for minority men vs women, 14% vs 12 % for white/asian men vs women). The rate of unwanted sexual attention is drastically higher in the tech industry than other industries (10% vs 6%), but the rate of unwanted sexual attention reported by women is only slightly higher than by men (10% vs 8%). For bullying and harassment, white/asian women reported a lower incident rate than white/asian men (15% vs 16%). But minority women reported a substantially higher rate than minority men (13% vs 9%). You'll also notice minorities reported a lower harassment rate than whites/asians.
I highly recommend reading the actual report if you're curious about this stuff. It doesn't really fit into any of the stereotypes (hah) about male/female or white/asian vs minorities.