I would presume that without a traveller, and using vang sheeting (with stretchy vang) as a substitute, when going to a beam reach .... that the main became overtrimmed at the foot, and grossly powered-up in the mid panel sections due to boom-rise. This characteristically happens to jib/genoas (and most staysails) as they are not set to travelers.
I used to have the same problem with vang sheeting coupled with a lousy 'pin stop' traveller. A proper max. length traveller helped solved all this 'power-up' on a reach ... I wanted fast not powered-up; a rising boom / stretchy vang can cause such a 'power-up'
On my sport boat I have a full-width of beam radial traveler for the same reason .. and the 8:1 vang simply could not do the job of keeping the boom at a constant angle to the deck when the boat was 'screaming' on a reach.
Next time out in such conditions, walk forward and measure the boom to mast angle when the helm becomes heavy ... if much less than 90 deg., you'll need a good traveller to correct the situation.