Two possibilities I can think of:
- The tank was using DPS-oriented combos instead of high-enmity combos. He would generate less enmity than he might otherwise, and if he isn't being defensive he'd have to be healed more, causing more heal enmity.
- Overhealing. In this game overhealing causes threat the same as regular healing. Spamming heals when they aren't needed generates a lot of unnecessary threat. Better to keep the tank close to full health but not aim to fill him up with every cast (unless you know a big hit is coming).
The only thing DPS might have contributed to that situation would be to get hit a lot by AoE or enemy skills, since that would force the healer to heal them and generate more healing enmity.
