When an individual or group of people intentionally show up, uninvited, to the same restaurant/location as another (unknowing) group or individual with the intention to piss off the unknowing group by (hopefully) sitting directly next to them.
Steve and Jenna made a reservation at Dee's restaurant for a nice romantic evening. Steve mentioned this to his friend Bobby casually during the day. Bobby decided it would be funny to get a group of friends together and also dine at Dee's restaurant that night. Bobby's group of 6 arrived before Steve and Jenna and were seated. When steve and Jenna walked in, they were surprised (and a little pissed off) to see the uninvited guests snickering at them as they were seated close by. Steve and Jenna got Dinnerbombed!