I concur with much of what Bill said.

Also, if it was a CAT TL642, it does not have a rear stability cylinder. For that matter many 3 ton machines do not have a rear stability cylinder to lock the rear axle when the boom is raised. Most telehandler accidents in the US (not sure about EU countries) are lateral tip-overs as you describe. Front outriggers only slightly reduce the lateral tip-over risk.

See graphic at constructionequipment.com/articles/images/CEX/20060201/cex0602_herobL.jpg

As you raise the boom the CG moves up and back, making the machine easier to tip. The rear cylinder turns the "triangle of stability" into a "box of stability" which greatly reduces the chance of tipping. My guess is the operator used a jib attachment pointed almost vertical, with the rear of the machine downhill.
ISZ