Look up the related thread about Tank on E.
The core idea is to be able to get in and out of tank when you need it. This gives bastion some needed mobility (especially vertical mobility) while also adding depth and unpredictability to his kit.
There are other changes proposed in the very same thread, all of them are dependent on one another to be balanced.
This would make Recon his long range hitscan form, sentry his medium range DPS form and tank his mobility/burst damage form.